The easiest thing to do is to remember (i.e. save to disk) the user's selection of either node-locked licensing (TA) or floating licensing (TF). This way you don't have to "figure out" which they're using -- you already know.
If you don't want to do that, then you can use TA_IsGenuineEx(), and if that fails use TF_RequestLease(). But honestly, it's cleaner and you'll be able to provide a better user interface if you just remember the user's selection. It can be as saving a single byte to file.