EASIEST quick test is.........
1..........Twist key to start and hold it......wiggle shifter from park to neutral. If you get a response, the NSS is going bad, or linkage out of adjustment, or the wiring connector is loose on the NSS
2..........Make certain it's in park or neutral. Go out and disconnect the quick connector on the starter relay which goes down over the transmission. Take a clip lead and ground that terminal, see if it cranks. If so, NSS problems as in 1 above
If no crank in 2 above, do 3 below
3........Do Red's test above, which is determining if you are getting "start" voltage from the key
4.......If you have tried 2 and 3 and it won't crank, it likely is a bad relay. LISTEN for the relay click. Then jumper across the relay exposed large terminals. This proves the wiring to the starter is OK. Replace the relay.