Convert input file or stream to UTF8 and detect if it's UTF8 or not (asp.net)

Convert input file or stream to UTF8 and detect if it's UTF8 or not (asp.net)

what would be the best way in C#/asp.net to do the following: - detect if
file is reliably UTF8 or not; - convert input stream or just file on disk
to UTF8 programmatically.
Cheers Askar