A conformal coating used to be typically, like a layer of bees wax or something a bit harder. It was hot-sprayed or dipped or painted on the completely assembled circuit boards and allowed to dry. It certainly COULD be a layer of spray paint
or polymer coating these days. And depending on why it was being used, any one of them might be "the best" for a given application.
But in any case, if you have good young eyes, or reading glasses, (or, there's a group "Magnifying light" application for smartphones that turns them into a magnifying glass!) all you have to do is take a good slow look at how the components are mounted onto the circuit board. Especially any jumper wires or bits that are soldered onto the board. If the bare metal "legs" on each device are BARE METAL, it was not conformally coated. If you can take an X-acto knife or other blade and scrape some coating off the metal parts
that were soldered onto the board? Then it was coated. And probably touch up the scrape with some clear nail polish if you're really worried.
The only time conformal coatings get critical, is when they cause components to overheat (bad design) or when they throw off capacitance on circuit boards, a common problem in early high0frequency and computer boards, again, bad design. Circuit boards have to be re-tested and often slightly redesigned after a particular conformal coating is applied, and then retested again if the coating is changed to anything somewhat different.
Not harm to check for them, though. And if you've got a spare board, and it looks fairly simple and not terribly heat sensitive...sometimes easy enough to add one.