It's actually pretty easy to determine if it's the v-card/system or the modem.

If the framerate is low, i.e. the pictures just run slower and it starts looking like a slideshow, it's the graphics card, or the ram, or the cpu...

If the game runs smoothly, but then you suddenly get your character reset in 5 metres behind or the enemies seem to teleport from one spot to another, it's the modem.

I've only seen one acception to this, being Splinter Cell Chaos Theory Coop Mode. There the modem problems looked like you having a weak system.