How do you measure rear axle width?

I ordered a rear wheel from online store and it appears my 142 mm axle is 5-6 mm too short for this hub when I try to screw it into my frame.

Which immedietelly made me suspect this is 148 mm boost hub.

If I insert this wheel into my gravel bike 142 mm frame, the brake disc sits at correct position. Also on cassette side everything seems ok ( but I cannot screw down the axle.)

I checked the hub which came with wheel to see if maybe some sealed bearings werent pushed all the way in on the cassette side, but everything seems ok.

Brake side rotor is in correct position like I mentioned.

Does this mean all the width is in the hub itself, not the spacing around the hub ( 148 mm boost wheel hub is wider than 142 mm hub?)

I dont know how to "prove" this is boost hub.

Wheel is Gurpil infinite 6b disc tubeless.
https://www.gurpil.com/es-ES/Catalogo/Index/RUEDA-GRAVEL-DISCO-6T-12X142-SHIMANO

Yes, its says 142 mm but like I explained my axle is 5-6 mm short for this wheel and have been using 12x142 wheels on my bike for years.