How would I know if my HeNe laser tube is at end of life?

Try checking the high voltage with a CRT voltage checker. If the HV is there, the tube is probably bad. I've repaired a number of HeNe lasers through the years. 90% of the dead lasers were dead tubes.

I think the question didn't have to do with a tube that was dead, but one that was weak.

A tube rated 1 mW is considered end-of-life when its power drops below 1 mW at the spec'd operating current with proper mirror alignment.

In other words, the power rating is the minimum output power within the tube's spec'd lifetime.

However, mirror alignment is often an the cause for low power and that can be corrected.

(But if this was a tube from a Laserdisc player with an output of 0.8 mW. I would expect that alone not to result in the player failing to operate correctly.)

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