If the horse has never run on an off track, you’re best to look at the sire profiles and siblings. Personally I use the overall TGI as a baseline and compare the off track to that.
If the horse has run on off tracks, your best bet is to look at how they’ve preformed on those surfaces. Did they run an outlier number compared to their form at the time? How did they react off of that? ... The key thing is asking yourself the right questions and assessing the performance on the sheet to find the answer.