Unfortunately there's not a uniform rim marking standard as there are uniform tire marking standards. Sometimes the maximum air pressure is stamped on the rim, either on front or back side, and sometimes inside the rim. Sometimes there is a tag stuck to the rim with the information. Sometimes there is nothing.
If your rim has a manufacturer and model on it, you could contact the manufacturer and ask. My fifth wheel came with load range D tires - 65 PSI maximum inflation on Dexstar steel rims. When I decided to upgrade to load range E tires at 80 PSI I confirmed with Dexstar that I had 80 PSI rims.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch