Ok sorry if this is a dumb question, but if I have an address I sent coins to, is there a way to tell how many total coins are in their wallet.dat?
If a subsequent transaction spending those funds also includes other inputs, you might be able to conclude that the recipient also has the funds from the other inputs as well. That is not guaranteed though as if the wallet is a shared (hosted) EWallet, it will combine inputs which may be funds received from numerous unrelated transactions.