How to Locate Pet Urine Stain?

Dog urinated on carpet and it evidently wasn't discovered. How to get the smell out? Not worried about the carpet itself as it is old. Any special carpet cleaning machines?

We've been using the spray successfully (Simple Solution Stain and Odor Remover), but am betting this incident soaked into the carpet pad. And lastly, how do I locate the spot? It's somewhere near the hallway - and short of getting down on the floor and sniffing (which I don't want to do) - I'd rather do something else. Do those black lights work? Where to buy?

Help!

Holly from Richardson, TX