I don't believe that there is any notation on the title that states that an SPCN is SB100 Smog exempt or not. The best, 100% fool-proof way to tell is to see the SB100 Sticker - on the sticker will be the vehicle's VIN, which can then be matched to the title.
So for a vehicle you cannot see first hand, tell the seller to photograph the SB100 Sticker and the title, as both will (should) have the same VIN.
Additionally, you can probably contact the DMV (go to your local DMV) with the VIN number and ask them to look it up in their compooper.
It used to be easy to use the DMV/BAR web site to find out, just by entering the VIN or License plate, but I can't interpret the response when I entered my VIN/License plate -
DCA BAR Vehicle Smog Test History
Hope this helps,
Randy...