Excellent article by Paul Graham on How to do what you love.
[the] new definition of work [is] to make some original contribution to the world, and in the process not to starve.
I think the best test is to try to do things that would make your friends say wow.
If you admire two kinds of work equally, but one is more prestigious, you should probably choose the other. Your opinions about what’s admirable are always going to be slightly influenced by prestige, so if the two seem equal to you, you probably have more genuine admiration for the less prestigious one.
The test of whether people love what they do is whether they’d do it even if they weren’t paid for it