You can not tell how some stranger plays until you are betting money, there are several things to look for to tell how good he is , even when he is laying down!.
I always felt that if I didn't know who some pool player was, i had the best of it, until he showed me different!
Gary abood came in our pool room, nobody knew him, so we started playing even 1 pocket, for 50 a game for awile, after being even for a couple of hours, we had the bet up to 300 a game and then he showed his speed , i know i couldn't win even, i was up a couple of games then he got even and i quit.
Of course after that match i called a few friends and found out his true speed, i was told i needed 9-7 10- 8 to get there, Abood comes in the pool room a couple days later and i told him i will bet high if you played me 9-7 and he said no but i will give you 10-8, so we played a race to 5 for 2000 me getting 10-8, i won the set 5-4.
We played a year later at 10 -8 for 2500 and he beat me 6-5.