No instructor has ever mentioned a red line and I don't recall reading about it in the C152 hand book.
Try the "limitations" section of the POH. Under "engine".
I did not find a full C152 POH online quickly, but stumbled onto this thing which is an (uncertified) summary:
http://www.ginecoweb.com/Piloto%20Pr...a152Manual.pdf. Page 3. 2550 RPM.