Well, yeah, I gave two properties, one possibly non-destructive, one destructive, that could be used to find that out. Obviously, the destructive test only helps if you have spares to sacrifice.
Apparently, failure isn't a good indicator anyway, as a 20V gate might pop at 30V to 80V.
Probably safer and more reliable is to just assume what voltage it is. Low Vdss transistors usually have low Vgso ratings. I don't think I've seen anything over 30V that wasn't rated for +/-20V gate.
Tim