Well, you can technically "test" fuel pressure anywhere choose, but like Cab says, at the top end after the system's had to keep the bowls full under WOT is where you'll find the system's limit.
Basically, you need enough pressure under that kind of demand to keep the jets fed & the engine from leaning out.
Some carbs' needles & seats will only tolerate 4.5 PSI - lowest I've seen was 2.5 PSI on a stock flathead Ford - and the Holley ballpark is around 7 PSI.
Those #s are easiest to achieve under low loads, but not so much under real demand.
I like to take that measurement as close to the carb as possible.