What are the things that could damage a glow engine, or shorten its life, and also symptoms that would show that the damage has already taken place. For starters, here is what I know :

Things that might damage :
1. Running to lean ( how much and how long ? )
2. Too less oil content


Signs of damage :
1. Lack of compression ( most obvious )
2. Starting problems
3. Sluggish throttle performance ?

Please add to the list..

There is also the possibility of bearings going bad which manifests as a jumpy/notched feeling when you slowly turn the engine.

Corroded bearings will - usually - also be audible as an additional "whirring sound" when the engine is running.

QuoteThings that might damage :
1. Running to lean ( how much and how long ? )
2. Too less oil content

Both will have the same effect.

Quote1. Lack of compression ( most obvious )
2. Starting problems
3. Sluggish throttle performance ?

1. Yes
2 & 3. Could be several other causes for these symptoms.