This is a collecting genre that is fraught with peril, many corps badges are post war, made for GAR and reunions, others are outright fakes. Unless you have a very tight provenance and know what you are doing, it is best to shy away from this area.
There are things to look for, such as pin length on metal badges, but this is a field that one should tread in after researching, researching and then doing a little more research.
Cloth corps badges have been added to surplus caps and sold as original to the piece, which doubles the value of the cap. The implication being that is was a war worm cap and not surplus.
Cloth corps badges were almost always sewn on and will leave a bit of shadow underneath, if possible, lift a small section to see if it has been recently added. Soldiers would invariably sew these on themselves and many are simply "tacked" on with few stiches generally penetrating just the wool and not the paste board. They were tacked usually to supplement a brass number or letter that anchored the whole.
Corp badges are also found on sides of caps as well as the top, this example shows just that, with a replaced chinstrap (notice slit for button):