(Answer assumes that you've picked code bases from a couple of projects that are relatively successful and well used).
If you mean the best technical solution then you can't but there are lots of definitions of right.
If this is working code from a popular application then that in itself is a reasonable definition of right, and that's one big part of what you can learn - you can see how people have solved problems in the real world and trying to work out the compromises and trade-offs they've made (and perhaps why they've made them).
The other thing is that you can see how people have solved particular problems you might be interested in so pick a piece of functionality you're curious about, or one that's relevant to the sort of programming you do or want to do, and start digging through it.
But I'd definitely try and focus what you're doing. Skim reading code will teach you very little. Pick an area you want to understand, or find a bug or change you'd like to make and focus on that. Just reading code is a passive activity and for most people that's a fairly ineffective way of learning.