How will you know if a guy is serious about you and your relationship?

Introduce him to the hottest chick friend or acquantince you know: his type preferably, if you know what he likes. Find a way to get them alone together. Ask her to try and get him to fuck her. She can find some way to pull out of it, if he goes for it. Or some back up plan.

If a dude turns down pussy, it's serious.

Ehh. First thing that came to mind. Of course, you could also just spend more time with him and discover it on your own, naturally, rather than being kinda devious as my suggestion. Does it really matter if he's serious? Just enjoy the moments you have together, without thinking about all that, imo. And you will know eventually.

If he is given a choice between his favorite thing or thing to do... or spending time with you or doing something for you.

Find out how he acts when you aren’t around.

Because you won’t question it.

Lol no they can't do it