For starters: A little known fact is that the Model T was not originally offered with black paint. The early 1909 models were red and gray or dark green. During December 1910 and January, 1911, the dark green in turn was changed to a dark, almost black, midnight blue. Finally, in late 1914 to early 1915 the blues were replaced with just plain black on the open cars. From this point on, black was the standard color until the "improved model" was introduced for the 1926 season, with green offered for the Tudor and Coupe bodies and Maroon for the Fordor body.

Nevertheless, the vast majority of model T's were black, and by 1924 half the cars in the world were model Ts; it isn't hard to imagine that one might easily forget which car was his. Fortunately, there were many ways to make one's T stand out from the others.

1. License plates.
In 1903, Massachusetts passed the first law requiring automobiles to carry registration plates; by 1918, every state in the U.S. required them. These plates, originally made out of porcelain, were an easy way to identify your car, provided you remembered what the plate number was.

2. Accessories.
There was an enormous market for accessories for various cars; significantly larger than there is today. The ubiquity of the model T meant that aftermarket suppliers could reach a very large market with just a single product. Despite the warnings of the FoMoCo pamphlet Don'ts for Ford Owners, which advised that

The Ford car is made right and equipped right [. . .] when it leaves the Ford factory. Investigations have shown that where "troubles have come" in the running of Ford cars, they have been caused [. . .] through the use of outside devices and unnecessary accessories,

Aftermarket bodies, accessory radiator caps, tops, radiator shells, and wheels(to name a few of the more than 5,000 various devices advertised in publications like Ford Times) flooded the market alongside knick-nacks and doodads of doubtful utility (like these "anti-rattlers" which are just heavy gauge wire twisted into a spring, or an accessory starting mechanism that claimed to allow you to start your T like a lawnmower [in the top right]), all of which served to "personalize" the owner's T and helped it stand out in a crowd. In addition, automobile clubs or associations like the Automobile Association of America or a state-specific counterpart would distribute plaques to their members that could be attached to radiators, another way to make your car stand out. E.B. White (author of Charlotte's Web and Stuart Little) wrote in his essay Farewell to Model T that

when you bought a Ford, you figured you had a start - a vibrant, spirited framework to which could be screwed an almost limitless assortment of decorative and functional hardware."

3. Custom work
The cheap price of labor in the 1910s and 1920s meant that paying someone to give your T a new coat of paint or a custom body was within the price range of a number of T owners; a customer who really wanted his T to stand out would not have had to travel far to find someone capable of doing that sort of custom work.

I should point out, however, that most people left their T's in (mostly) stock condition. Apart from their license plates, there really wouldn't have been much to distinguish two T's of the same body style and model year apart. However, the particular quirks and peculiarities of each T were endearing to owners, and many became quite attached to what was more often than not the first automobile (or any large piece of machinery) they owned. For them, the particularities of their car were as clear as day: maybe the car inched forwards a little on starting; maybe it had a particular dent or scratch somewhere; maybe one of the brake shoes dragged a little bit; maybe the engine note was just so. In any case, most owners would have been able to distinguish their own car, much the same way as I can identify my car when it's beside others of the same model and color.

Edited for sources:

Casey, Robert. The model T: A Centennial History. Baltimore: Johns Hopkins University Press, 2008.

Sears, Stephen W. The American Heritage History of the Automobile in America. New York: American Heritage Pub. Co. :, 1977.

White, Elwyn Brooks. Farewell to Model T. New York: The Little Bookroom, 2003.

I don't know of any specific examples, but it's worth noting a few facts.

While the Model T may have been the most common vehicle on the road for much of the 1920's there were more than 10 different variations of the Model T available with varying door numbers, styles, and other options. In addition, for the first few years of it's manufacture it was available in four different colors.

Lastly, the Model T chassis was often sold to coachbuilders who could manufacture unique and different looking body's for the vehicle. Personal customization was also not uncommon to allow the Model T to fit into a variety of situations and personalities.

Even then, if you confused two similar vehicles and could not remember where you left your Model T, there were also 24 different keys for the Model T. While it may have been possible to jump in another persons car, the odds of you being able to start it and drive off were rather low in the case of a mix-up.

Steinbeck, a contemporary of the Model T, writes about the Model T often and gives the reader the feeling that each driver knew his Model T quite well, from the look and feel of it, to the way it would crank. While they would be considered simple at the time, you would likely find it exceedingly complex to simply start compared to a modern car.

Two generations of Americans knew more about the Ford coil than about the clitoris, about the planetary system of gears than the solar system of stars.

All that said, even today we mix cars up. I once got into a 1986 model Toyota Camry identical to my own via unlocking the door with my key before realizing my mistake.