Ditto will always appear as another Pokemon. Usually it will appear as a common Pokemon. A good way to tell is if a common Pokemon seems really hard to catch its probably a Ditto.
Ditto will always be disguised as Pokemon the following Pokemon:
Zubat Pidgey, Ratatta, Gastly, Mankey(I think)
Sentret, Yanma, Hoothoot,
Zigzagoon, Gulpin (and 1 other Gen 3 Pokemon I forgot).
I may be forgetting some from that list.
I find Dittos on Pokemon go a lot.