Background check for freelance work?

Just got an offer from a startup, they're doing reference/background checks. Have been freelancing on and off for the past couple years, listed that on my resume plus my favorite project which spanned part time for 4 months. Is a reference/call to my latest client enough to verify that? I made in the 100s so I didn't even have to pay taxes, so no w2/1099. Also instead of listing Upwork (the service I have mainly worked through) , wouldn't I list self employed on the background check form they're giving me? Basically this is my first full time out of college and idk how freelance work is verified

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