I absolutely agree that your first attention to these guns is to verify that they are not loaded.
If you've never fired patched round ball (PRB) before, I highly suggest you start with either a damp type patch lube, such as Hoppes Black Powder Solvent and Lube, OR if you use some other type of patch lube such as bore butter or animal fat, then just swab between shots. Otherwise it is easy for fouling to build up and you wind up with either a PRB stuck in the barrel that you can't get all the way down on top of the powder, or the jag, cleaning patch, and rod stuck in the barrel. Ask me how I know.
If you swab between shots it is a good idea to then pop a cap before loading again to ensure the flame channel is clear and you didn't push fouling down into it.
I don't know your level of experience, so disregard if you are fairly knowledgeable.
1. Have a few different thickness patches. I'd go with .490 RB and have .010" thick cotton, .015" thick cotton, and .018" pillow ticking for example. Start with the thin .010" and if it's too loose or groups aren't great, go up to the next size. If the .015" are fairly tough to push down the bore, DON'T bother trying the .018". You can also collect shot patches about 10' out from the bore and examine them for how they are holding up. Badly burned up, holes, tears indicate too thin of a patch, or poor patch lube application, or old patches if pre-lubed store bought, or a burr or something cutting them in the barrel.
2. Start at 50 grains of powder. Stay with this load until you iron out what patch thickness loads the best and has the best accuracy. Then start upping the powder charge by 5 grains at a time and shoot groups. So, shoot a 3 or 5 shot group at 50 grains, then do it again at 55 grains, then again at 60, etc. etc. until you find the best load the rifle shoots. Don't exceed the max load per the rifle manufacturer. You'll likely find it somewhere between 70-90 grains.
3. If you load the rifle and then only get a pop out of the cap but the main load don't ignite, especially after swabbing the bore:
wait to make sure it isn't going to be a delayed ignition fire. Remove the nipple and put some powder down into the flame channel, put the nipple back in, and try it again. It will usually go off.
4. Don't use too much patch lube. If you use a wet style patch lube you only want them damp. By damp I mean the feel wet but you can't squeeze anything out of them. If a bore butter or fat/lard style, then just apply enough to fill the weave of the patch. It doesn't take that much, but make sure it is rubbed clear out to the edges.
Sorry I wrote such a novel, but Dutch's recent post about first time muzzleloader shooting reminded me how poorly my first time on my own went. haha