It’s hard to tell which is which. Unless you can open up the subtitle track itself (some MKVTool apps let you extract single tracks) and look at the data in it, it’s mostly trial and error.
It’s often best to have a local computer video player (cough VLC cough) to play the newly ripped file from your DVD. When you play the file, you can select each subtitle track one-by-one and see it play right then and there. Once you determine one track that IS ENABLED but doesn’t seem to be showing any subtitles at all during normal speech, you most likely have found the “forced” track. Remember which track was the forced (for example, was it the first subtitle? second?).
Then, load the file into MKVTools. Remember which track was the forced track from before? Mark that - and only that - as forced. It does not need to be default, and it does not need to be first. MKVTools will spit out a copy of the file with the flag now set for that track. Get rid of the old file. Put the new file to where Plex can see it. Plex Dance it if you already added the movie to Plex previously.
Yes, it takes a few extra steps every time you rip a video. Unfortunately, DVD/BR makers did not design them with the intention people would later rip them to a digital file, so we have to add these forced flags ourselves every time.
(FYI, I think some programs can tell you the FILESIZE of each subtitle track. Forced subtitle tracks are pretty much always going to be much much smaller than any other kind of subtitle… Often close to 1% the size of the rest. If your ripper program shows the file size of the subs, look for that info.)
