Also look carefully at the pictures for evidence they were taken in the UK. In this case the plug sockets are not UK, so you are less sure that this is the person you are going to meet.
In my experience there's not one pattern or one surefire way of knowing, you add up the evidence and make a decision.
At the end of the day the best thing is to go ready to walk if it's not right. Easier said than done, but it's your best option.
"I'm really sorry but I've left the money in the car" is a reliable get out. Not nice, but it works.