How do you know when to replace tubes

What I do is buy two sets of tubes for my pre-amp --- using one set as if they were my only tubes and the other as back-ups and to test for the degradation of my primary set of tubes. After about a year, I swapped out the tubes that were in for a year for the other set (that were the same type & stock and had maybe 10 hours on them). I played with the second set for a week and determined that the 1st set with a year on them sounded the same. So I put the 1st set back in and played them another year.

At the year 2 mark I tried again the back up set and realized that my 1st set (with 2 years on them) sounded a little grainier and had a little more noise and did not sound as "lively." So... I deemed the 1st set to have lived the life out and trashed them. My back-up tubes then became my primary tubes and I buy a new back-up set.

A side note... I have had tubes blow on my power amp and cause much damage. I have entirely gone away from tube amps and do tube preamp w/SS amp.

Finally... tube rolling --- if you have the $ and the time to play around can be anything from fun and advantageous to kind of a waste of time.

A tube tester is always a good way to tell.

In our gear, it tends to move towards a sleepy sound if the tubes are shot.

I am just a moderate tube user these days, but first thing I check for is any audible noise or hiss. Also if anything sounds unusually different in any way, tubes are one of the first suspects. I keep a few spares around so that I can a/b regression test individual tubes at a minimum and replace if not sure.

Noise is easy to identify but other sonic anaomolies not so much. A good quality tube tester like Atmasphere recommended is probably the only way to be sure. Not as common anymore these days, but depending on where you live, in lieu of owning and using a tube tester yourself, there might be shops nearby that keep a tube tester around and might be able to test them for you.

I would replace the two 6550 in the power supply after 2000 or 2500 hours. It is cheap insurance against the possible damage a blown power tube could do. The 6h30 tubes are not going to do any damage if one fails.

I also have a complete set of backup tubes for all my tube equipment. That way I always have a spare in case of failure.

BTW I own an Audio Research Ref CD7, Ref 110, LS27 and PH8