You want to use the Date function in your find to calculate a date six month previous and search on equals to or less that that date, so it might look like this
Set Error Capture [ On ]
Enter Find Mode  // Deselect Pause
Set Field [ TABLE::ContactDate ;
Current = Get ( CurrentDate ) ;
" ≤ " & Date ( Month ( Current ) - 6 ; Day ( Current ) ; Year ( Current ) )
Perform Find 
If [ Get ( FoundCount ) = 0 ]
Show Custon Dialog [ "No record found" ]
And this will also display a message if no records are found.
I hope this helps
Brilliant, it does what I needed it to do. Thanks a lot for your quick help!