That definitely sounds like a leak. The temp gauge won’t move because losing a little coolant isn’t going to change the temp. But that small leak can turn into a big leak real fast, and when it does, the car is suddenly out of coolant and overheats within a couple of minutes. Usually happens while people are driving and they don’t notice until it’s too late.
There is no good way to check. If you take it to the dealer, they’ll put a pressure tester on it. Might or might not show a leak - mine did not. They generally will not investigate further. For the front crossover, you can try peering down into the “V” of the engine with a flashlight and look for orange colored residue - there‘s a tiny space near the oil filter. Or, if you have a little borescope, you can shove it down there. But it’s still not definitive and the rear crossover is really tough to see.
If you’ve got about 70K or more miles on it, the best thing is to just have them changed out. Change the water pump at the same time - maybe the T-stat as well depending on your mileage. It’s not cheap if you have someone else do it, but sure beats breaking down on a road trip or, worst case, replacing the engine (which runs about $15K these days from what I hear).