How do I know I'm not starting off too fast on exercising?

I have asked a similar question based on how slow I should start off exercising again; and I must confess that I did not follow my plan/ goal at all. I am yet again attempting to start exercising, seeing as the year is coming to a close and such. My question is, is there a way to tell if one is exercising too hard in the beginning? Like, can people count their heartbeat and know that it’s too intense for starting out (vice versa)? What’s a good start/ routine for a lazy person like me to begin exercising again? (I’m 17 years old)