Using an older UTH

So im seriously considering purchasing a leopard gecko, and Im going to buy a 15-20gal tank for it. I was wondering if it would be okay to use my brother's old UTH heater from when he had a couple of anoles about 9 years ago. The heater is a 6"x8" zoomed repti therm U.T.H (120V 60Hz 8w). I have it plugged in right now, and its decently warm, but im not sure if its warm enough. I wanted to avoid purchasing a brand new one since they are quite costly, but if i need too i will. How can i tell if this heater is usable?

Thanks in advance

Get a good thermometer, the digital type with a probe are best. Place the probe on the floor over the pad and if it hits mid to high 90s you're good to go.

and i was also wondering if that size will be okay? 6x8

That should be fine.