I assume you mean a context menu scan, a.k.a. quick scan, this by its nature is a quick scan to allow the user to get on with what they were doing and by default doesn't display results. If however the file is infected all hell will break loose in the form of an audible and visual alert, so you will know.
- To change the default settings - right click the avast 'a' icon, Program Settings, Common section, Show Explorer Extension Results.