If they often talk about the future and if you are in that vision as-well. So things like, I can't wait to have our own house together and stuff like that.
I mean the most direct way of finding out is by asking what they think of marriage and have that convocation
But just because there are no clear signs about it, doesn't mean he doesn't want to or isn't thinking about it. Perhaps he really does but may feel that you wouldn't want to so isn't showing the signs. So therefore the best advice is to talk about it with them : )