Excellent question. Keeping your ear to the ground and asking around probably.
Over the years I assume people will make spreadsheets comparing the relays and clients and those columns will be on there. 
For now, it’s about finding out.
For example, I recently contacted one of the major app-dev/relay-hoster about potentially-illegal content referenced in a post. They said they wouldn’t do anything about it because the content wasn’t hosted by them.