First is the obvious one that if it says V-Brake on the brake, it's long pull.

Second is that if you look at most flat bar brake levers there are two holes, or at least an indent where a second hole could be, where the part that the head of the cable can go into is (the silver bit in this photo: https://www.parktool.com/assets/img/repairhelp/housingU_005.jpg). If it's in the top hole, then it's long pull. If it's in the bottom hole then it's short pull. There are sometimes a little "C" and a little "V" next to those holes/indent that make it even easier.

If that second one doesn't make sense, I can get a photo later today and send it to you.

Third is by measuring the distance from the pivot point to where the head of the cable goes. Park Tool says that it is 21mm for short pull: https://www.parktool.com/assets/img/repairhelp/ta-flatbar-conv.jpg and 42mm for long pull: https://www.parktool.com/assets/img/repairhelp/ta-linearpull.jpg

In practice for the third method I tell people to measure with a ruler the distance and if it's less than 30mm then it's short pull, if it's more than 35mm then it's long pull and if it's hard to tell or somewhere inbetween then it's hard to tell without testing it.