As far as I know, your tax records are private. There are however surveys and a lot of unions ask members to disclose salary (with no personal information) so they give out "average wages" for specific areas, such as system development, web development, general programming, etc. (these samples were taken from one of the IT-focused unions). So it gives you an estimate of where your average should be and you can work from there.

I still haven't been aggressively asked by employers what I make now, but I have been asked aggressively what I wish for (as in, you tell us first and we'll say yes/no/maybe).