What Mode?

Hey guys, this might be a really dumb question but how do you tell what mode you are playing in? What I mean is if I'm playing in G Ionian what makes it G Ionian and not say for instance A dorian? Any help is good help.

Thanks,
Toe-Knee

Well G Ionian and A Dorian have the same notes. the difference is generally where you start and end and which notes you emphasize