I would like to practice using ATC for approach in the training server on Live. How do I know which airports have ATC on the map so I can practice approach?
The airport code turns green on the map when there is ATC (in some form) there.
Also, in the select menu you have a list of airports by (no of people flying there) along with any active ATC services.
If you’d like here is a visual aid to emphasise what @BennyBoy is saying
Also if you click on the airport, then click on its tag you can see which frequencies are open
If you’re still unsure what airports will have active ATC or approach for that matter, you can always count on KLAX or EGLL.
LiveFlight also has the info if you want to prartice under specific controllers :)