I think you can also tell by looking at the shuttle carrier - though that also could be converted . . .
I read online somewhere, that Singer started putting the high bobbin winder on the last of the serial numbers allocated to the 27, so you may find one from the factor with the high bobbin winder that has a number listed as allocated to model 27.
I'm tempted to go pull mine out and see if there are other differences underneath. I think I only have one 127 though, and have too many 27's