Many of my perennial garden plants have been getting eaten by some animal. Possible culprits include deer, rabbits, and groundhogs. I would like to control them, but I have to know what I am controlling. for example, I don't want to start trying to control rabbits when deer are the problem. How can I find out what is eating them so I can match control to culprit?
As well as looking for droppings and specific damage, there are number of other options depending on how much time and/or money you have:
All three of these are used by the professionals. If you are so inclined (ie. make your own electronic gadgets), then motion detector cameras could be quite a hobbiest DIY project.
If you have soft soil around the plants that are being eaten, you can figure out what the animal is by looking at the tracks left behind -- the difference between a rabbit and a deer will be obvious.
Height is another differentiator. Groundhogs, for example, are unlikely to eat the tips of the branches of your apple tree that are 4' off the ground. Voles might attack from underground.
Droppings are the last way I know of to figure out what is eating plants. Last summer, for example, I thought I had a woodchuck sneaking through a gap in my fence and nibbling on my peas. But when I patrolled the fence looking for the gap, I found skunk turds.
If you find tracks or droppings that you aren't familiar with, a google image search will usually turn up something useful.