Very hard to do without leaving evidence. A couple ways to tell:

#1 Hold the encased up to a bright light and look for traces of light shining thru.

#2 Signs of glue or other adhesive residue around the cent. If the original was removed the replacement cent won't stay in the collar.

#3 Does the date of the cent match the date of the Casino. Example: Joe W. Brown's Horseshoe having a 1980's cent.

#4 Does the wear on the cent match the wear on the collar?

#5 Try to remove the cent w/o damaging the collar. For most encased cents this is almost impossible to do.

I have an encased cent that looks like it was run over by a train. The original cent is still tight in the collar.