I don't think its that hard to look for evidence of units growing collectively tired of battle. In the case of the 1st/95th I found the following indicators.
1. A considerable rise in desertions. During April 1809 to December 1811, there were only three desertions. During November 1812 to May 1814 there were dozens. We know who the deserters were and how long they'd served from the casualty returns. I dont have my notes in front of me, but to my recollection the deserters were all or nearly all old soldiers of ten or more years' service.
2. Disciplinary problems. The Rifle and LD more generally incurred Wellington's displeasure on a couple of occasions during June 1813 to April 1814 for thieving and other crimes. I do not believe they were similarly reprimanded during 1810-11. We know that these crimesincluded at least one case of murder.
3. Fraternisation with the enemy leading to the surprise of 10th December 1813 on the Nive and capture of 13 soldiers. This event was regarded as a sign of war-weariness and slipping standards by several observers. My own view also is that the willingness of those 13 - 'old soldiers' of the Highland Company - to go into captivity was quite different to the behaviour of soldiers, for example at the Coa in 1810, when quite a few of those captrued managed to escape.
There are other areas which would be worth examining too, for example the number of soldiers listed sick at certain times.
all the best