How do you tell if your....

[road rage]

Running rich? And how do you lean it out?

 

[FallenPhoenix]

gas mileage will be low, you may see some black smoke from the tail pipes if it's really bad, you may start running rough due to buildups, I'm sure there are others but those come to mind first.

Best way to fix it if you're heavily modded is with a tune, otherwise little things like a CAI and full exhaust should help even it out a bit.

 

[twogts4us]

Running rich? And how do you lean it out?

Get it on a dyno.
But since you're running nitrous, you're better rich than lean...