All in all, only around seven hundred purebreed PA-11s were ever produced and while there are many more flying, most are converted J-3 and it is difficult for the untrained eye to distinguish them. However, while very similar, there are some differences:
An original PA-11 has al slightly wider and more stable landing gear. Upgraded J-3s carry the same landing gear that they had as J-3.
Flying a PA-11 requires the mastery of the art of hand propping.
Converted J-3s have a windshield that is sloped at a rather different angle; this is due to the fact that the J-3 nose is longer than that of the purebred PA-11, since in the latter model the fuel tank was moved from there and placed on the left wing. Besides this, the angle at which the engine is mounted in a J-3 - converted or not - is slightly different than the PA-11 specification. In other words, converted J-3s are "almost" PA-11s.
Another picture showing how to start a PA-11 engine; notice the fire extinguisher.