I sightread through the whole piece and toy around a bit with the parts that seem difficult. It's not always 100% correct but it does a decent job in identifying the problem areas in a piece.
I have also read some of Bernhard's threads on this. However Bernhard also makes the point that you don't put anything HT until HS is complete. So should one initially making sure the whole work is safe and secure and up to speed HS and then do the sight reading through to isolate the difficult HT bits to start on?