I think you will just know. I had the pulling of having one more after my 2nd child, discussed it with the man and at first we were both not sure...so much to think about going from 2 to 3 (being out numbered and all :)...we thought about it for a couple months, then both decided we wanted 1 more. Went for it, had our 3rd and then both were ABSOLUTELY sure we were finally DONE! I had my tubes tied right after my having my 3rd and am grateful we were both in agreement and happy with our choices.
~IMHO, I think when you are not sure if you want 1 more, then you probably really want 1 more...when there is that part of you that is thinking about the what-ifs and all, ya know? But when you know you are finished than you just know it with your WHOLE HEART!
Good Luck! I suggest you and your man talk about, put it on the back burner and see what happens...worked for us!