Verifying income - How this this accomplished?

[Jesse Kerr]

When underwriters verify income, how is this accomplished? - aside from just looking at what I sent them.   Let's say I forge all the numbers on my tax returns, w2's, paystubs, etc. etc.. would they be able to catch this or verify that what I submitted is false?  From what I'm reading, they go off of what you submit to them. I know how it sounds, i just can't seem to get an answer online anywhere.