Not exactly scientific, but level the bike (on the ground), then jam a big piece of plywood up against the tires, then lift it till it hits something (pegs, floorboards before engine guards), put a protractor on the ground and measure the angle of the plywood, and you have your lean angle.
As stated, different shocks (13-11.5") and springs and pegs and floorboards, and peg lowering kits and tires (OE, and others off-size, and car tires), fork rake kits, will change the max lean angle.
It probably also makes a difference if the bike is riderless, or one up, or two up. (load)
Measuring it after you let it fall over doesn't count (even if you slid/rode it 50 feet that way).

do u remember the member on here yrs ago that used his valkyrie for track riding school and showed pics of the worn down crash bars from leaning over so much?