Wheels

Hope this doesn't sound dumb but I am going to look at some LWB rims, how do I know they are LWB ?
Any obvious things to look for ?

I think those are the "Almost round" ones.
A bit like the very old threepenny bit. ;-)

Assume you mean around the hub ?

LWB are 5.5J, SWB are 5J
If they don't have tyres just measure them.
Tubed or tubelss?
Avoid 5J, too few ttyres to fit.
Google the part numbers if you can read them. They may have different numbers but be the same wheel, that's just LR.
Check the offset is the same,just look at them, the worst thing is buying a set with one odd wheel.

That is a good point, make sure they all the same !