There's no definitive answer to your question simply due to the fact none of us can inspect the rifle in question for condition and possible damage. First thing to do is drop the rod down the bbl (keeping all body parts away from the muzzle of course) and mark the rod with a piece of masking tape and comparing to bbl length. (by laying the rod alongside the bbl with tape marker even with the muzzle, other end of the rod should reach approximately to the nipple location. IF its substantially shorter the weapon could well still be loaded.) to assure yourself the rifle isnt loaded. OR if it is to safely clear the charge and projectile.
Is it clean? or was it put away dirty after being fired? Pull the nipple and inspect. is it clean or dirty with primer/powder fouling? A snug cleaning patch wet with HOT water on a rod run down the bbl should give a good indication the rifle has or has not been cleaned. Assuming it has been cleaned the patch should come out fairly clean, though perhaps with a bit of rust residue if not properly dried and lubed. A bore light can then be used to look down the bbl and visually inspect for corrosion/damage. A small LED light is availble that can be dropped down the bore lighting the bbl from the breech end.
If inspection shows no problems then more than likely its safe to shoot starting with light loads and working up a little.
Another option is take it to a qualified gunsmith for inspection.
As far as proper cleaning, hot water is the tried and true method. A modified nipple with a piece of plastic tube is availble to replace the stock nipple. Drop the tube into a pail full of HOT water, and pump a cleaning rod fitted with a swabup and down the length of the bbl repeatedliy untill all powder residue has been removed, then start dry patching the bbl until patches come out clean and dry. By using hot water, this heats the metal up and helps with drying. Stand the rifle in a corner with the muzzle down for an hour or so, dry patch again to be sure no moisture remains, and run a lubed patch down the bbl coating the metal as a rust preventative, being sure to dry patch the bbl prior to shooting the next time.