A good trick is to take a gallon jug, fill it, and pour it into your empty fermenter. Then, take a sharpie and mark that level. Repeat several times.
Now, you have a really good idea as to about how much beer you have. You'll lose a half gallon or so to trub, depending, but you can eyeball that and get a good idea.
Mix your priming sugar based off of the guesstimated amount of beer. If you're alittle over or under, odds are that you will never notice.