BBF, the most exact pressure sign available to us is the case expansion at the pressure ring measured with a four place micrometer. It's a real PITA and I won't do it except in those very rare cases when I am working around absolute max.
Primer reading is the most common and the most useful, but you have to have used one brand of primers long enough to have a feel for what a given degree of flatness means and it does not work at all in stuff like the .45-70 where you are way over the redline before the primer starts to flatten at all.
One huge red flag is tiny metal shavings around the firing pin hole. A burr there can give you a false alarm so I polish that hole slightly on my guns. It that area is not burred, even tiny shavings mean that the primer has been pushed into the hole. That's way too much pressure. On bolt guns, a bright spot on the case head where the ejector cut was means the same thing for the same reason.
Sticky extraction in a Handi is a sign of overly high pressure if you have eliminated the other common culprits.
Far and away the best method is to stay away from loads so hot you need to worry about excessive pressure. The rule of thumb is that it takes a 10% pressure increase for a 5% velocity gain. That last grain or two of powder is unlikely to give you enough extra velocity to be worth the pressure increase.
A chronograph is a real handy tool, too. It'll tell you what a one grain increase is doing to velocity and it'll tell you when you have the velocity you need. If you get up around book velocity and your charge is still well under listed max, it's time to stop. If you get to the listed max, and your velocity is still low, it's time to stop anyway. Try another powder.
All this stuff takes experience. You have to get a feel for what your guns do. Best way to get that experience is to settle on safe working loads and load and shoot a few thousand rounds before you start worrying about max performance. (If then. Been there, done that, ain't going back.)
