A finger should be able to pass thru the hymen. I remember feeling a hymen in my youth - ages ago - the membrane and the central hole quite distinctly. So I would have thought that you could feel if it was there or not. Pain - it seems to vary. Some girls say they felt nothing, others say it was agony and everything inbetween - I would guess it also depends on how "ready" you are - how horny, how wet - and I would guess how big he was! Various object have been used to get rid of hymens in preparation for the big event - vibrators, fruit etc. Messy cums? Can happen, but usually no big deal - we could pass any accident off as a wet dream! I would not worry too much about that.
Have fun, Starlight