I just bought a little '95 Civic CX w/ an aftermarket intake (yes, it's riced right out.. previous owner like a loud car, but the thing is gutless).
I have a little 16 valve 4 cylinder 1.5L motor in it. (manual).

The car idles fine, but the intake is making a slight whistling sound (which I heard is normal with aftermarket CAI) and there is another noise. I'm
thinking it may be a vacuum leak because I checked the spark plugs and there was heat damage to them (they where all white), and also the car
seems to be runing a bit lean.

The noise itself is coming from the intake manifold area and seems to be a very low sounding 'woosh'.

How does one check to see if there is a vacuum leak?

 

I would suggest spraying it with a light mist of water and watch for bubbles, but that only works for tires, because the air is going out instead of being sucked in. What you could do is get a very very light, flimsy piece of saran-wrap, and float it over the intake where the sound is coming from. The saran-wrap will be pulled towards the leak, unless it is a very small leak that does not suck in much air.