I'm wondering if there is a website or something (free) that would let me see how many seats remain in a given class on either previous or future flights? Or whether the seats were booked originally or people upgraded to them the day of.

The reason I ask is I am looking to choose a premium economy seat on different airlines and if one has more frequent unsold business class seats then perhaps I have the higher likelihood of paying to upgrade at a discuount at the gate.