If they are big African marigolds I don't know as I've never left them to seed. Ordinary calendulas will set seed easily and there is no mistaking the seed heads, like round knobbly discs. They go brown when ripe. Don't leave too many or you will have them popping up everywhere!

if they are the african/french type dead heading will encorage a second flush. true for the english pot marigaold too, thinking about it. dead head and see if you get more. you can always leave one plant alone for it to set seed now if you are worried.