Determining existing wire's specs

I'm installing a flourescent light fixture to replace a single bulb fixture. The light's installation instructions says that my supply wires need a 90 degree C rating. The house is older, and the fixture warning said older homes may have wiring rated for 60 degrees C. The existing wire in use is a white NM 12/2 W G, with a copper ground. There is another flourescent lighting fixture in the house, and the supply wires all seem adequate, but ...how can I make sure that the existing supply wire's rating is 90C, and not take a chance on starting a fire? Much thanks!

 

Sorry, but your wiring is not 90-degree wire. 90-degree wire is marked "NM-B". Your wire, marked "NM" without the "-B" is not 90-degree wire. You only have to replace the last couple of feet of the cable with new cable to eliminate the hazard. But rememeber, all splices must be made in permanently accessible junction boxes.