In trials, I size up players by looking at gear, emblems, and grimoire. There isn't much else you can do besides that. I've played against max grims and they sucked, I've also played against low grims and they were a fair match. There isn't really any sure way of knowing unfortunately. Although, if they are using unleveled gear and anything beside thorn, it's typically an easy fight. (Looking forward to the meta change, regardless if it's for the better or worse)