if the string is typed in A1 then something like the below will do it:
hopethis helps
J

Sub FindColon()

Dim SearchString, SearchChar, MyPos

SearchString = Cells(1, 1) 'range("A1") contains the string
SearchChar = ":" 'Search for ":".

MyPos = InStr(1, SearchString, SearchChar, vbTextCompare)

MsgBox "The colon was character: " & MyPos

End Sub