The descriptions of the 95th, 49th, 45th and Belem's Rangers that Mark and Ron provided are great examples of something, depending on what you are looking for.
The behaviors of the 49th and 45th sound a great deal like the typical activities of the 88th, which was known for stealing, drinking, outrageous conduct, "the 'deadbeats'--dirty, malingering, and
untrustworthy"--and with it's excellent combat record, combat fatigue doesn't seem to fit--or does it?
The Belem's Rangers are interesting to, because officers could leave on 'personal business' any time they wanted. It drove Wellington to distraction, and he complained about it to Torrens all the time. Why would officers be part of Belem's Rangers when they could just go on leave? I would imagine they were the poorer men who couldn't afford to go home. So, what prompted the creation of 'Belem's Rangers?' incorrigibles?
So, from the descriptions you have the following individual behaviors:
1. lack of personal hygiene, appearance and personal care
2. Dishonesty and theft
3. Avoiding duty, particularly combat duty, lazy or cowardly
6. attempted suicides, self-inflicted wounds
7. refuse to fight or inventing reasons to leave the fighting.
8. faking illness
9. riots, mutiny, and rebellion--refusing to follow orders
The two problems with these individual behaviors is first that any evidence of individual behavior is all anecdotal in dealing with the question of entire units exhibiting battle fatigue, and second, all the behaviors could and were seen in men and units that couldn't have been battle fatigued, or at least it was unlikely.
But let's assume the above list is the list of behaviors related to 'battle fatigue'. To use individual behaviors as evidence of battle fatigue, you would have to:
1. Show the individual behaviors being directly related to the stress of battle and long campaigning.
2. Demonstrate a connection between to the amount of time a unit was on campaign and the frequency of the behaviors. In otherwords, is there evidence that the longer a unit stays on campaign, the more likely it is to exhibit the behaviors above?
3. Identify contemporaries who recognized the unit(s) in question had lost at least some fighting capabilities.
Very often, instead of dealing with a difficult issue, a historian will ofter up an anecdote or two, a contemporary's observation, or an event that illustrates the conclusion the historian wants to make anyway, and that is it.
To deliver an conclusive evidence, there needs to be a functional description of what is being looked for, a concrete list of things that must be found to establish a reasonable conclusion, and finally a 'game plan', for finding the information methodically--as in 'with a method'.
I sometimes feel that the lack of solid evidence is often a lack of solid methodology, rather than the actual existance of evidence. I can remember reading several books on the Revolutionary wars where the author would bemoan the fact that there wasn't enough evidence on the small unit tactics and how early the French were practicing particular maneuvers. How many hundreds of examples did Lynn find for just 1794 and the Armee du Nord?