normally if a player talks to me in game and tells me they are going to go donate then its done pretty much right after you donated.. And I kinda know its you because its not like someone else would have just donated at the same time without previosuly saying so or anything.
Other wise I'll just ask you for the name that was used on paypal, to confirm it was your donation.
Then I sort out the rank & perms.
Theoretically I could get paypal communicate to the game server and promote you in game if you donate a certain amount, or setup specific purchase options from the donate page. However I'm inclined not to for a number of reasons, so I just do it the way I've described above and that works fine so far.