Easiest way to check if file is NTSC or PAL?

Open the "Movie Info" window while viewing the file with QuickTime Player.
If the frame rate is 25 it's PAL. NTSC uses 29.97.
If the file uses a reduced frame rate it has already been "converted" and it really doesn't matter what the original source used to be.

Now I'm even more confused by your question.
If the files are to going to be converted to DVD (MPEG-2) format what does Toast care about the source?
Most modern DVD players don't even care about the "source" format. PAL discs work just fine on most modern NTSC players and vice versa.
Are you fearing a "slowdown" of the 29.97fps NTSC files when viewed using PAL at 25 fps?
Or is the issue one of screen size (PAL is a bit larger). Your images are getting "squished"?
Since Toast converts to whatever source (PAL or NTSC) you wish to create you should only be confronted by this issue if a "DV" file is the "source" file. You say you're using .avi and .mpg files.
They are no longer .dv files.
As a test, use a short (known to be NTSC and PAL) source file in the old MPEG-1 format and "burn" these files to DVD using toast.

Toast 7 can accept combined PAL and NTSC sources for burning to a DVD. However, you need to select whether you want the DVD to be NTSC or PAL in Toast Preferences. Toast then will automatically re-encode the "wrong" format to be the selected format. This is in Toast 7 only.