Easiest litmus test I can think of is that gxp steps down from 24mm to 22mm on the splined side. On bb30 the splining starts without a step. Also the bb30 is turned for bearing contact in two spots of the same diameter, while gxp is turns for bearing contact in two spots of different diameters (and often on the splining)

Also the bb30 axle is... 30mm while the gxp is 24mm. That's 20% larger- it should be easy enough to eyeball if you use google image to calibrate your dead reckoning.

Post the pictures.