How to inspect/test old capped off drain pipe
I would start by pouring water down that drain. If you have a cleanout outside, remove the cap and have someone pour water down the drain and try to observe the water flowing. If it's been capped for a long time, which it appears to have been, you'll probably have to snake it as stuff has probably settled in that pipe. Again, observing down the cleanout, if available, helps. If all this investigating pays off, you can go with it. If you really need "peace of mind" you can rent one of those drain camera's for a couple hundred bucks.