If some evil developer want to steal my code and use it in a closed-source software, how could I be aware of it,
There are various tools that can be used to examine software binaries for indications of copying. For example, the
strings command on a UNIX / Linux machine will reveal literal strings (versions, error messages) in a native code binary.
You could run these tools yourself ... modulo constraints on reverse engineering that might be imposed by the binary's license.
Alternatively, someone else could "tip you off".
and how could I prove it since I don't have access to his source code ?
If you have a well founded belief ... and a good lawyer ... you could launch a breach of copyright lawsuit against the suspected IP thief. As part of the discovery process, your opponent can be compelled to release the source code to a court appointed expert. The expert would be instructed to identify evidence of copying.
(Historically, you could also assign copyright of your GPL software to the FSF, and they would take action to defend it. I don't know if that still applies ...)