your car might be lean due to the induction and mufflers I have similar mod and the got a dioblo tuner to tune the can for the mods I have. Money worth spent.
You may be able to get a bit of an idea from checking the plugs, but it won't tell you if you have rich/lean "spots" the best way to tell is to get a tune, but you could get a mail order tune to correct any issues.
if youre running too lean, youll often hear detonation at full throttle.. the motor will also run extremely hot if its severe (the exhaust will feel extremely hot). as for too rich (which is the way it usually goes), your exhaust will smell very strong and make you gag.. thats the biggest easiest ways to tell.
to find out for sure, go get your car on a dyno machine. they will be able to tell you your exact air/fuel ratio and give you options on how to correct it.
be prepared to spend a few hundred bucks to correct the a/f ratio properlly.
with the mods you have, your a/f is probably just fine assuming everything else is working right.