For little outlay I purchased a No 2 Dental mirror $4 and a single LED Booklight $2, this allows me to check the valve alignment in both the up and down positions.
This has been a revelation to me, the last two horns I had professionaly serviced, cleaned and re-felted came back worse than before they went in.
My recent purchase of a Mt Vernon Bach from a local dealer, serviced in his shop had a miss alignment of 2mm in the up position. Replacing the new felts with the correct size made a big difference in the performance of the horn, 10 minutes work.
Bob Reeves Precision Valve Alignment (patented) $220, is an exacting measurement, recording and correction process.
For a student or part time player the local tech alignment much cheaper will suffice if care is taken.
A simple test is to see how far a valve has to move before there is a change in sound or feel of a note.