How can I document that I Moated an enchantress?
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.