Follow these few simple step to locate membeer records that have not yet recieved a thank you:
From your Search menu, select Contributions.
Fill out the search fields at the top of the page if you'd like.
Scroll down to the bottom of the page:
1. Set Date Thanked to is NULL.
2. Click Find.
From here you will be able to see a list of members to whom you have not sent a thank you.
See our support document here for instructions on how to thank multiple records at a time.