If you overheat to the point where hardware may be damaged, you will usually get a BSOD. This is a hardware failsafe kicking in to prevent you from frying the components.

If you are always running borderline hot, it could have long-term issues. Actually the first thing likely to go is the battery as those are very susceptible to heat damage. You will also probably throttle, which you can see in Windows performance monitor or using a third party utility.

Generally though, if you are overheating frequently you’re definitely going to know. Once you know, it’s probably time to check your fans and paste.