Copper fouling can be hard to see, so the best "test" is to clean with a good copper solvent.
There are a number of these around and can be bought at most gun stores.
Some good ones are Hoppe's Bench-rest Copper Solvent, Shooter's Choice Copper Remover, Accubore, Butch's Bore Shine, and others.
Sweet's 7.62 solvent is a very effective solvent, BUT you can't allow it to stay in the barrel very long since it can harm the bore.
To "test" for metal fouling, just swab the bore out with a couple of solvent-soaked patches to remove the carbon and powder fouling.
Let the barrel soak for 1/2 an hour, then wet a clean patch with the copper solvent, and run it straight through the bore and out the end.
If the patch is stained with green and blue stains, this is a sure sign there's copper bullet jacket material still in the bore.
In this case, continue running wet patches down the bore and letting it soak between patches, until the patches start coming out without the blue stains.
The best way to keep fouling from affecting your accuracy is to properly clean the bore each time you shoot it.
Usually, the procedure is to use NEW bronze cleaning brushes soaking with solvent to brush the bore, then do the solvent-soaked patches until no traces of fouling are present.
Be aware, that the solvents EAT the bronze brushes, so clean them off as soon as you're through with them, and replace them OFTEN.
Another tip, is to buy a good stainless steel or synthetic-coated high-quality cleaning rod.
If you're shooting a bolt rifle, another method is to brush the bore with a brush, plug the bore with a chamber plug, and fill the bore with solvent.
Allow to soak over night, and the bore should be clean.