You still have to be careful about the non market app you install. Before installing google it to see if anybody has had problems with it or had a malware problem. If it's brand new, wait to see if there are any reported problems.
Avast and Lookout and Kinetoo: Malware apps that scan an app as it downloads, they can do a complete system scan of files as well. They can not tell you if the app is corrupt until it's installed.
Try downloading it to Your PC first as your PC won't auto install it. Then run a scan with your PC's malware.