Whenever possible, I go by the uncooked weight because I feel like that is more accurate. The weight after cooking will vary depending on how long you've cooked it for and what temperature you cook it at--the longer and hotter you cook it, the more moisture cooks out of it and the less it weighs. So 5 oz of hamburger cooked to "rare" would weigh more than if it were "well done." So if I'm looking at calories for 3 oz of cooked hamburger, I never know if that's really the same amount of meat as the 3 oz of meat I'm eating.
If there are calories stated on the package, they are always for the uncooked weight unless the package specifically says it is for the cooked or "as prepared" weight. So if your package says that 5 oz of hamburger is 190 calories, that's 5 oz weighed before cooking, unless the package says otherwise.
If you are looking at an online source, book, or other resource for the calories, the resource should specify whether it is for cooked or uncooked weight. For example, CalorieKing provides calories for ground beef
raw,
pan-fried, or
broiled. When I look at those three pages, I'm pretty sure my 4 oz of raw ground beef is the same amount of meat and water as their 4 oz of raw ground beef. But I can't say the same thing for their cooked ground beef because I don't know how longed they cooked it or at what temperature.
And sometimes it's impossible to go by the cooked weight. If I'm making beef stew, I pretty much have to weight the meat before I cook it. On the other hand, there are also times when I have no choice but to weigh the meat after I cook it. If a roast a whole chicken, for example, I can't weigh it before I cook it because it includes bones, skin, and fat that I won't end up eating.