Unless you have remote access to someone else's computer, you probably won't be able to tell who installed software on a given machine. However, there are a few things you can look at to try and figure it out.
First, check to see if there is an uninstaller for the software in question. If there is, it's likely that the person who installed it wanted you to be able to remove it easily, which suggests they were the ones who installed it in the first place.
Next, take a look at the file permissions for the software. If only a select few users have access to it, that's a good indication that whoever installed it didn't want just anyone to be able to use it.
Finally, check the software's documentation to see if there are any clues there. Sometimes developers will leave notes indicating who installed the software or when it was installed.
If you still can't figure it out, your best bet is to ask around. Someone else on the machine might know who installed the software and can tell you.
