Is there any easy way to tell if a big brake kit will fit 17 and 18 inch wheels?
No.
Can you tell just by looking at the offsets offered or do you have to contact the the dealers and ask about each and every wheel type?
Even that might not work. The problem is that a wheel might clear the calipers around the OUTSIDE (the rim) whereas the SPOKES might not clear the caliper. And vendors will not know for sure unless they have a customer that has actually tried it with the same brake configuration on their NSX. Barring that, the only sure way to know is to either ask someone else who's done it with the same brake kit, or to try it yourself.