I made 2 alignment rods out of drill rod I bought at ACE hardware. Got a piece of 1/4" and a piece of 5/16", went through all they had to find the straightest piece. I cut them down to about 16" length and went to a machinist buddy and got him to turn down about 3" of each rod. He shaved the 1/4" down to .224 and the 5/16" down to .308.
After that, I took the rods home and chucked each in my drill press and used emery cloth to polish the diameters down some more until they were a slip fit into the bore of my 223 and 308. I now have straight alignment rods that are long enough to enter the barrel a few inches and stick out of my suppressor. I screw the suppressor on the host barrel and stick the lubricated rod in the bore through the suppressor. If the rod will not enter the bore or the rod is not centered in the endcap of the suppressor, then the threads/suppressor are out of alignment.
I had a 20" ar bbl threaded once and the threads were not straight with the bore, but the face of the cut-down barrel was properly perpindicular with the bore. When I screwed my QD adapter onto the barrel and torqued it down against the barrel face, the face of the adapter and the face of the barrel met up at a slight angle. When the torque was applied, the two faces corrected their alignment, but in the process the crooked threaded portion of the barrel was pulled over straight. This pulled the actual bore out of alignment for the last 1/2" inch of the barrel.
POI shifted to 7 o'clock about 31 inches from point of aim at 100 yds. It still grouped well, and when I removed the QD adapter the barrel sprung right back to proper zero. If I had just went ahead and installed the suppressor without checking with my alignment rod, I would have probably initiated an "undesirable event", likely destroying my expensive suppressor and pissing me off to no end.
Later I had to get the bad threads cut off and the barrel re-threaded, winding up with a 19" bbl rather than 20". This time I sent the barrel to YHM, the maker of my suppressor, to get it threaded. That way if it wasn't right then YHM would still cover any suppressor damage.
TJones75 has the right idea as well, and that would be much easier for a one-time alignment check.