I put a stickie note in the back with the date that I last finished
When enough time has passed, usually 5-8 years I have forgotten enough to reread. The older I get the less time it takes.
My 4 favorites to reread:
When worlds collide-Balmer and Wylie
The Stand-King
Lucifer’s Hammer-Niven and Pournelle
The Longships-Bengtsson
by outta-touch