Say someone plays Enchantress and I react with Moat. For the sake of the question, it is a 5-6 player game and it would be easy to forget that I reacted. How can I easily make it clear and easy to remember that I reacted with a moat?
The moat says :-
When another player plays an Attack card, you may first reveal this from your hand, to be unaffected by it.
So you are not 'playing' the moat. To defend against an attack you merely show it from you hand.
Now if the moat is in your hand it's likely you have defended against an attack card so would just remember. If however you need a reminder you revealed a moat to defend you could do many things such as
- place it face up in front of you and pick it up at start of turn
- if you want to hold your hand look at cards you could could turn it back to front so moat is 'revealed' to other players
- you could just put your hand down and leave moat face up on top to show you defended.
The game its self doesn't have any markers to remind you you defended. But when it gets to your turn and the duration effect is in effect it shouldn't be to hard to recall you defended (unless you've got some player who tries to dispute it!). Ultimately I would always assume you've defended unless you want the attack to affect you.
Don't forget that even if you defended against on attack you can choose to let another affect you. ie you might find being attacked by Minion useful to cycle hand.
I'd also like to add the simple solution that if you forget that you showed a defense card that is in your hand, it's still in your hand and you can just show it to other players again.
As the comments below point out, this is not a 100% certain thing, but it comes pretty close. I'm afraid that I don't know enough about anything beyond the base game to catch existing exceptions but if you find yourself stuck in one of those exceptional cases, a method to track this sort of thing might be beneficial to agree on as a gaming group.