@Semioptera Thanks!
I had a hard time telling them apart too, but I believe the main difference is that Forster's are larger and have a thicker bill.
It's a very subtle difference, but the bill of this Forster's Tern looks larger and has more black on it.
@Semioptera The best way to tell is knowing that the zoo's last pure Forster's died in May 2014 and that the remaining four or so birds are all Common with the exception of a single CommonXForster's. My photos of the birds below:
