How do you test different tubes?

Andy at Vintage Tube Services has the real deal. The idea that there are no good NOS tubes out there is patently untrue. As Elizabeth indicated, many private sellers are offing ones they did not like, but the best dealers are getting ones through means others do not have.

The last place I would buy a tube is eBay FWIW. At least not at full price as you will need to sort them.

I just spoke with Brent Jesse (audiotubes.com) regarding NOS 6SN7 tubes. He has an impressive supply of very high quality NOS tubes of all types. He allows you to buy, try, and return for upgrade or refund (-20% restocking fee). Although I didn't end up buying from him this time (I found what I wanted locally), I plan on doing so in the future.

I concur that there are legitimate vendors as named above along with the Tubedepot & Tubemonger that you can get quality new in box NOS tubes, and at reasonable prices which normally to my ears sound a lot better than the new production.

I use Hickock 752 and a TV7DU tube testers, but I just got a giant Sylvania tester that I have high hopes for. That's how I test different tubes.

NOS tubes are definitely available. Here on Audiogon I regularly see people selling off NOS tubes from their collections that they realize they will never use.

There are crooks out there so you have to be careful. It's best to have a tube tester if you are going to buy NOS or used tubes and buy from someone who will allow returns.

If you don't want the hassle of learning about tubes and tube testers, the tube sellers noted above can get you top quality tubes for a price.

IMO, NOS tubes can make a difference in the sound of your system that is worth the price.