Hmm, I read this question slightly differently from Robert who correctly asserts that commented out code should be removed.
If, however, you are looking for a convention to mark code for later removal, an old favorite of mine is:
//b = false; //TODO: remove
Some IDE's flag
//TODO: comments or can be taught to. If not, it's usually a searchable string. It's best to follow whatever convention your shop has established because this can be done several ways. Every code base should do this one way. Keeps it searchable.
quickly parse which is which?
Without that mark the automated way to do this is with the compiler. If stripping the comment off produces code that compiles, it must have been commented code. Writing an IDE plugin that checks that wouldn't be hard. But it will leave buggy commented code behind.
This is why it's better to simply mark commented out code as code the moment you comment it out. This lets you work non-destructively while you decide if you really want it gone. Since we all get interrupted, and are somewhat forgetful, don't be surprised if some lines get checked in while in that state. If they do it's nice that they're at least clearly marked and searchable. Keyboard macros have helped me with this in the past. It's hard to get interrupted in the middle of this if you can do it with a single keystroke.
You can take this as far as enshrining the mark in your continuous integration tests. Oops, I'm trying to check in with outstanding TODO's again.