how can you tell if...

When I buy the shrimp from the supermarket, first, look at the shrimp, watch the feet and the color, the color of fresh shrimp is bright. Then smell the shrimp, the fresh shrimp has no unusual smell.

First of all, the smell of the shrimp will change after a few hours outside. So you can check whether it is still fresh or not is from the head of the shrimp. If the head is almost separated from the body, then it is not that much fresh anymore. The fresh shrimp/prawn would be the one with the still hard shell, with the full a bit hard head and not too much unpleasant smell.

I agree, check for brightness of color and no strong fishy smell! You may also notice what time of day it is when you buy it and whether or not it is frozen?