The phrase to be not so green as one is cabbage-looking, and its variants, mean to be less of a fool than one appears to be.

This phrase plays on two uses of the adjective green:
1) denoting the colour of growing vegetation, grass, etc. (cf. green fingers and green thumb);
2) denoting an inexperienced or naive person.

These are, in chronological order, the earliest occurrences of the phrase to be not so green as one is cabbage-looking and variants that I have found:

1-: From the caption to the following cartoon, published in Sydney Punch (Sydney, New South Wales, Australia) of Saturday 11^th March 1865:

BUSH SLANG.
Scene: Interior of Hut up the Bush.

Old Hand—“I wish you’d get up and light the fire, New Chum.”
New Chum—It’s all very well calling me a New Chum, but I’m “not quite so green as I’m cabbage looking.”

2-: From a letter to the Editor, published in The Caithness Courier, and Weekly Advertiser for the Northern Counties (Thurso, Caithness, Scotland) of Friday 20^th October 1876:

We are very far north indeed, sir, but “we are not so green as we are cabbage looking.”

3-: From a correspondence from Manchester, Lancashire, England, by ‘Bumble-Bee’, “a Workman in the Centre Cell of England’s Great Industrial Hive”, published in The Chicago Daily Tribune (Chicago, Illinois, USA) of Thursday 11^th September 1879:

The deathly pallor of most Manchester people sometimes assumes an almost greenish hue in its very intensity. Still, we are not so green as we are cabbage-looking; and, were we not to wash our faces once a week, whether they want it or not, we should soon be soot-black, and smoke-dried into the bargain.

4-: From the East Aberdeenshire Observer (Peterhead, Aberdeenshire, Scotland) of Friday 1^st September 1882:

The venders of vegetables in town have a grievance. Those of them who pay shop rent and taxes, and their brethren who have to “cash up” at the rate of 8d per diem for a stance in Broad Street find that they cannot compete with their “country cousins,” who, with their “shop on wheels,” make a house to house visitation, or even make a stand in Broad Street, and other thoroughfares; but who pay no tax because so long as they do not unyoke, no tax can be levied upon them. Nor do those cultivators pay for a hawker’s license. As a consequence they are enabled to sell their produce at a lower figure than town dealers. Hitherto the greengrocers have borne this wrong with resignation; but now they are taking active steps to have it redressed. They are about to shew the community that they are not so green as cabbage looking, for a petition, praying that these country dealers shall either be made to take out a hawker’s license, or pay a tax for privilege to sell their garden produce, is being signed by the vegetarian fraternity for presentation to the magistrates. The dignatary [sic] who is bold enough to come forward as their champion at the tilt in the civic arena, will be sure of, at least, a sumptuous repast of “hotch-potch” from the voluntary contributions of grateful vegetarians.

5-: From Stray Notes, published in The Southern Argus (Goulburn, New South Wales, Australia) of Saturday 30^th September 1882:

A splitter who was prosecuting his profession asked a silly looking shepherd who had come on the scene if he had ever done any splitting. “Well,” drawled the S, “I did a little on it once.” The hero of the maul then queried, “Which would you sooner be at splitting or shepherding?” “Well,” drawled sheepy, “That ’ere all depends.” “All depends” said the splitter, who was trying to burst up a log he had just sawn off, “All depends” he repeated thinking thinking [sic] within himself what idiots the majority of people are. “All depends” he asked once more, trying to enter the wedge which, however, not being fairly hit, turned on his hand, and thereby caused his old sap-stained mauley to receive a nasty jar. His patience not being improved by the accident, he demanded hotly “All depends on what?” “All depends” then promptly answered sheepy “On whether I could hit straight or not.” The splitter bending over his work mentally confessed to himself that sheepy could hit hard and straight enough. The moral which the splitter extracted from the experience was to the effect that a man is not necessarily green because he is cabbage looking.

6-: From the Wigan Observer (Wigan, Lancashire, England) of Saturday 4^th August 1883:

A LONDON BITER BITTEN.

A correspondent sends us the following:—It may not be generally known that within your district an extensive trip to London has just been made, patronised principally by railway employés. One incident connected therewith is worth recording, showing, as it does, that occasionally a man from the country unaccustomed to “city professionalism” proves himself fit to leave his native sod. The individual alluded to has always been considered scarcely up to par (to use a county phrase) in his top garret, and learning that he intended joining the excursion his comrades thought it wise to warn him of the many “traps” with which the metropolis abounds. He was specially cautioned against the pickpocket, who, if he was not careful, they said would relieve him of his purse. To such advice he apparently took little heed, though from what transpired it is evident it was not without its influence upon him. Quietly, and unknown to his friends he hit upon an original safeguard. Previous to commencing the journey he made a purchase—two pennyworth of fish-hooks. With these he carefully lined his trousers pockets freely, planting them all hook downwards. The train arrived at London, and amongst the excursionists was the man who had previously been designated “weak headed.” Our country friend, however, sauntered out of the station, and gazed about the thronged street and at the busy multitude. Presently his eye caught sight of a crowd, whither he was at once attracted. Almost instantly he discovered someone’s hand in his pockets. Having every confidence in the efficacy of his plan he was in no way anxious about his purse. Presently he moved away, and of course the “sharper” was obliged to follow, though the man from the country never appeared to be cognisant of his presence. Seeing the state of things a gentleman made up to our country friend and said, “I say my man, this fellow has his hand in your pocket.” In the coolest manner imaginable came the reply, “Well, ler him ger it eawt.” Knowing he was caught, and having a regard for his tortured fingers, the would-be pickpocket implored forgiveness and begged to be released from his painful and uncomfortable position. Being of a generous turn of mind, the country excursionist agreed to let the fellow off for the modest sum of sixpence—the cost of the fish hooks and a new pocket—and at once commenced the operation of cutting away his pocket. The London sharper’s agony provided fun for the groupe who witnessed his capture, and on once getting free he was soon out of his sight. The united opinion of the countryman’s friends is “that he is not so green as he is cabbage looking,” an old proverb in which the London pickpocket doubtless fully concurs.