There's no easy way to do it unless it's day of, or at least a few hours after 1500 HBT (when RAPs should be assigned). I use a combination of the Enhanced Reserve Display (ERD) and HI33.
There is no way to tell for sure what somebody is going to be assigned until ~1500 day prior because LC can be converted to SC and vice versa depending on the needs of crew scheduling.
Viewing the Enhanced Reserve Display is the easiest and most user friendly way, but the LC vs SC function sort doesn't work, so you need to look at RAPs to see who is LC vs SC. If you're more than a day out you can go to the HI33 and in bold red letters at the bottom of each pilot's section it will display SHORT CALL in red letters or LONG CALLib green letters (colors are on the APA TASC/SABRE site or app, which I recommend over DECS). It will also display the 2xxx for short call and 3xxx for long call similar to 3XP that was mentioned.
If you're using HI33 after 1500, you can sort by RAP, which will remove everybody who doesn't have a RAP assigned (including short call converted to long call or who haven't been assigned a RAP yet). This is essentially the same as looking at RAPs on the ERD.