Some careful inspection and common sense should be a good guide.
If it looks like junk, it well may be! If it looks like it's just a bit neglected -try cleaning it up.
Run a patch down the bore. If it comes out rusty go a bit farther and run a brush through the bore with a good bore solvent.
Is it very rough? Is it rough all the way? If it's not too bad some cleaning with an abrasive might get the rust off if it's just surface rust. If it's pitted, or if it's much worse in one area (commonly the chamber) it might be best to clean up the exterior and hang it as a "display", then go buy a new one to shoot.