Having the advantage of being employed in the composition department of a small newspaper, I used the RIP at our disposal to produce some broadsheet-sized proofs of my font. By setting up my PDF to be reverse, I got my letter-forms in black on transparent. The resolution is insane! It's also very handy for comparing shapes non-digitally, because I can lay multiple transparent sheets over each other.
Anyone tried this or have any other ways to proof that others might want to know about?