How to determine the "doneness" of salmon?
Press the salmon. The less cooked it is, the softer it will be. You more or less just have to learn by touch.

Broad rule of thumb: 6oz (flat-ish) portions of salmon take about three minutes to sear one side in a hot pan, flip, throw in a 350 oven for 5-6 minutes.

If roasting from raw, should be 11-13 minutes in a 350 oven.