signs that tubes need to be replaced

No, there really is no reliable way of telling in advance. If the tubes are used a very long time they can sometimes simply get weaker. Dynamics and extension suffer. But these changes can be so small and gradual over such a long period of time you may not even notice.  

If you are concerned at all then the smart thing to do is just order another set of tubes. Put the new ones in and if it turns out sure enough the old ones were shot you will know it right away. Save the old set for backup. Or if they sound about the same do whatever you want. If it was me I'd run the old ones as long as they last, maybe comparing them against the new ones every once in a while, until either one goes or they start to sound weak. Either way you keep your backups so you're never SOL waiting for tubes to come.

Drifting bias is a sure sign however that could also be caused by an out of spec bias resistor.    I had a matched quad of  EL34 power tubes that were about 8 months old.  One had "fire in the bottle" so to speak.  Literally arcs and sparks in one tube... it was scary.  Fortunately Quicksilver amps are robust and it only took out the mains fuse.   That was during play , it was sounding great and then all of a sudden horrible cracking noise through my speakers, I look up and fire....   I was lucky.  That said, I always try to replace before failure.  I have a set of KT88 that have a lot of miles on them and have been my spare a few times.  They still sound ok and hold bias but I'm nervous due to hours on them

I have a new quad of Gold Lion  KT77 on the way.  Hopefully they are as rugged and long lasting as they say

@jchiappinelli 
Look here for additional signs...
https://www.thetubestore.com/how-to-tell-if-your-tube-is-bad

@oddiofyl 
Interested in your update after you fully burn in the Genelex GL KT77s in your QS amps.  I'm recommending them to a friend for his QS mono 60 apmps in place of the stock JJ KT88s.