In exactly the same way any business owner does. Bring in your taxes from the last 3 years in order to get a loan.
I only deal with one bank, so getting a line of credit is really easy becasue they have listings of all my securities anyway. They see how much I pay on my quarterly installments to CCRA, and how much I invest in my RRSP's, so I didn't even need to provide any proof of income.
A non secured credit card was a little trickier for me because I applied for it just a few months after I began escorting. Therfore, I only had a couple months work with my 'new job' reported on my previous years taxes. I could have easily secured it with my bank investments, but I chose to just take a small credit limit to begin with.