Paid services Expertflyer and KVS Availability tool will give you availability in booking classes, which is more reliable that seat selection.

Generally if there is availability in A class for premium economy or J class in business before departure (both being the regular most discounted categories in those classes), there is a good chance of the upgrade proceeding.

If it is U class and C class respectively (the highest fare categories), the chances are slim, but an Elite recognition upgrade has the best chance of grabbing a remaining seat.

As mentioned already, a dummy booking requesting multiple seats will also help indicate availability, if you can't book more than 2-3 seats in the class you want to upgrade into, it indicates there is little space.