Original Message

I don't tell a nurse how to start an IV. That said, if I see blood leaking, after the nurse leaves, then I will bring it to their attention. 

You could just politely ask. It might save the pilot's life. After all, we are all human and daily stress hits us without knowing it. I did a year of human factors classes. So, yes, do bring it to their attention politely or find out what Standard operating procedures (SOP) are. I personally always do a full preflight regardless, even if someone completed it just before me. 

I have included a topical accident report. The first pilot said it was ok to go, and the second pilot seems to have just accepted the previous pilot's subjective decision. Guess who died?

https://www.ntsb.gov/news/events/Documents/2020-CEN19FA072-BMG-abstract.pdf