When a character is allowed to move to the next combat round while still holding 3 or more cards, cards that ignore the hand size limit like Officer's Harness should auto reveal. Not only will it let the opponent know that the game isn't bugging out or opponent cheating, but anything causing abnormal effects on the game state should be easily identifiable.
The best idea was probably pengw1ns about having a special icon for it. Revealing it is bad for the sake of cards that care about that and would make the card worse which really isn't fair.
Since it is the only card at the moment that lets you keep greater than two cards at the end of a round (as far as I know), I don't think there is a problem with it. If I see someone end the round and keep three cards, I know they probably have Officer's Harness. It may pay to do some forward thinking about it as there might be other cards in future that allow you to keep extra cards at the end of a round, but honestly I think that will be part of the strategy of the game. In every game I have ever played, high level play requires familiarity with the possible cards your opponent could be holding. I don't see how Cardhunter is any different. I'm not sure how someone is likely to cheat without hacking Cardhunter's servers since everything client-side has to pass through the server to get to the other client. As for bugs, I can understand that, but I think most people are likely to be honest about it and report the exploit pretty quickly. Some people will take advantage of it, but the exploit WILL get noticed and closed.