How can you tell when tubes are shot?

It is a gradual decline. If you have run them 18 months 24x7 I would definately replace them. If anything you do not want to risk an all out tube failure in the preamp that could damage something. I will replace the ones in my LS-15 at least once a year if not a little sooner, and it is only on 8-10 hours a day. Good luck, I am sure you will be surpised at how much the sound has changed when you put the new ones in.

It depends on the tube and whether or not you are running both the plate and filament voltage. Most units have a standby, that keeps the tube warm, but not on. This can extend the tubes life quite a bit (assuming it's in standby 24/7 and on much less). It also depends on the tube, many NOS tubes can last much longer than current tubes. Mullards are noted for their long life (although I've found them to be a bit "tubey" sounding). Siemens are also long lasting and I really like them--very transparent sounding. If I remember correctly ARC charges a real premium for you to buy tubes from them, so I would recommend a different source. Upscale Audio, Tube world are two I've used with good results.

You may not notice the slow decline. You may be use to the sound. Since you will need new tube eventually, get some and see if it sounds better. If the same, just save the new tubes until later.