If i didn't pay for a seat for my 1 year old and he well be on my lap can I check in a carseat

How much is fee

Dang, that's a spoiler! Throw it out and buy a new one!

They check things like that...usually at the gate. Its free that way. If you check it at the main counter they could charge you.

Carseats cannot be a carryon item. You must check it in either before TSA or gate check it (which has a fee)