How can you differentiate an aldehyde from a ketone?
The Tollens test is probably the most useful discriminator. It certainly is the prettiest.
An aldehyde can reduce silver nitrate to give a carboxylic acid, and (in the best circumstances) a silver mirror coating the bottom of the test tube. Ketones, as they cannot be further oxidized, fail the Tollens' test. Mind you, it is a fact that cyclohexanone does give a positive test.