Well, I agree about the static port problem, but you could also try stalls at the same altitude and heading (therefore same wind) using the GPS speed.
I'm sure we are all clear about the difference between stalling speeds in steep turns, stalling speeds in a straight line, and stalling angle of attack. For clarity, I was referring to stall speeds in a straight line. I can't imagine it would be fun or necessary to validate an angle-of-attack meter for slips.