Does the PC power up? If so, put in a HD disk and see if you can read it.
If not, you can usually tell the difference between 360K/1.2M 5.25" drives from the access LED color. Red is usually 360K and green/amber is usually 1.2M. I'm sure there are exceptions though.
AFAIK there are no visual tricks for differentiating 3.5" drives. 720K drives are often taller or "funky" looking compared to 1.44M that everyone is used to. But there are normal looking 720K as well.