Brian
There is only one way to find out if you're any good or not. Tune a piano and ask the customer. Ask the teacher if he/she's happy with your tuning. If she's not, ask her for advice. Be honest and upfront that you're starting out and appreciate any help you can get. Some will be happy to give it to you. Others might not, but don't let them get you down.
Don't be afraid of failure. Even after 45 years I still get an occasional customer who wasn't satisfied with my tuning. But most of the time it had nothing to do with the actual tuning. Some customers just like to complain, even if there wasn't anything wrong. I've learned that you can't please everyone. So don't let the few bad apples ruin it for you. Do the best you can.
Good luck, and welcome to the PTG.
Wim