Any idea how long an open can of Thai shrimp paste can last in the fridge? (How would I know if it's gone bad?)
Forever. But seriously, it will last a really long time, like 2 to 4 months. It's color will likely change if it starts going bad.
I would agree with Amanda, but only with the qualification that any food sold in a can that's been opened should be transferred to a jar or plastic container before refrigerating. Storing it in the can itself will impart an unpleasant taste.