What are the best ways today in order to learn about finding use-after-frees while code auditing?

I’m a novice CTF player with a decent understanding of OS internals and software mechanisms, I have deep understanding of heap and OS and User locking mechanims.

I would like to learn on how to properly identify use-after-frees in source codes. The classical examples of alloc-free-use are too simple, (at least for me), in order to deeply understand how use-after-free mechanims is actually identified. I’ve been reading about the subject for quite a lot in the past few weeks but I do not seem to find good examples, (bugs/writeups), on.

What are good examples, (bugs/writeups), or methodologies in order to properly identify use-after-frees in code?