[Phemist]

One possible way is looking at the list of editors of the Journal, and seeing if the researchers in question have back-mentioned that Journal on their CV (or personal homepage). The fact that researchers/peers in a field are willing to openly connect their prestige to those of a journal's, is usually an indicator that there is at least something to the journal.

Most if not all scammy journals will fail this test.. but it is also quite labor intensive.