Im off to view a second hand bike tomorrow. The bike/forks are too small for me (mountain cycle rumble with pace r36 forks), but the wheels, gears, brakes, crankset are all listed as XTR. My plan is to rip all the components and put them onto my sc blur.

What should I check for?

I think I would ride it to check brakes work and gears (front and rear mechs change smoothly and stay in all gears) etc. Punch it up a steep hill to check for chain slip/skip. Scrutinise the brakes and rotors for bent bits and damage. I suppose you should check the pedal threads and the O/S crank arm thread/spline on the chainset (scrutinise the spline in the crank arm, not the spindle/axle). Visual check on the wheel rims and spokes and freely spinning wheels with no gritty sound or feel in the hubs. I suppose the thing is to spend most time on the bits you are most interested in and those bits that are the highest value.

prod, poke and check for lateral movement on bits like mechs, forks, wheels etc to see if pivots/bearings are shot. Cracks on welds /deformation on frame.
If the chain is shot then the cassette and possibly chainrings are as well.

Mlke, the OP says the frame and forks are not important and of no interest.

But, like mlke says, maybe take one of those chain gauges, to see how worn the chain, cassette and chain rings might be. You’re looking at hundreds to replace those if they are well worn.

Frame number – check it has one and take a note of it. Then check to see if it’s stolen.

If it’s cheap enough for you to not worry about the frame and forks I would definitely be doing what epicyclo says or at least get some proof that the guy has bought it all legitimately. Then do the other checks as ^^

Still worth checking everything in case you find a crack/defect on something even if you don’t want it in order to haggle, or is that just mean if you don’t actually want the bit…
EDIT- no, it isn’t mean after some thought.