Crooked handlebars. What do you do?

One minor annoyance I have with bike work is, when re-tightening a stem,
getting the handlebars adequately straight. I eyeball carefully, but
often find when riding that they're a couple degrees crooked.

I suppose the task is more difficult on most of my bikes, which have
fenders and handlebar bags. It's hard to sight exactly where the wheel
is. And I find it harder with classic stems than threadless setups,
since the stem height wants to change as I adjust.

So, any particular tips?

--
- Frank Krygowski