99% of the time I have seated gas checks with my thumb, but the unsolved mystery of the occasional flying gas check has prompted me to be a little more careful, at least with "serious" loads.
Here's a homemade seater that I'm currently using on 30 caliber checks. Nothing fancy, just a disc that sits in the lubrisizer nut, and counterbored approximately 0.33". The counterbore is not absolutely necessary but it ensures that the base of the bullet is centered in the press. Set the bullet in the counterbore, then pull the handle down just hard enough to seat the check -- you can "feel" it seat. With a good fitting nose punch, this step also does double duty to "point" the tip of the nose.
Looking at the bottom of the seater.
Seating a check.
This batch of bullets is getting sized in a push-thru die prior to oven treating. I like to apply checks before oven treating, though some people do it afterwards. Yes, the push-stem was made from 5/16-24 threaded rod. Sometimes I make push-stems from 5/16-24 bolts, other times I turn them from bar stock, just depending on what material I find laying around.