Use your favorite file manager to find files and folders created or modified since a certain time. You'll be amazed how many it are, so it isn't a really good tool, but it's the best there is.
Maybe better give your friends only access to the guest account in your PC? Then they can't install anything.
Is it possible to find any changes to the computer? For example, added new files or programs that known or unknown. One of the users (friends or family members) or by any possible means it will be added or installed the programs to the computer when the computer has been used to connect Internet (intentional or unintentional) so how can I find it?