Perhaps invisible wizards can be smelled somehow.

Or certainly heard, unless they are also using sound-reduction charms.

There are also super-sensitive metal detectors, assuming most wizards might be wearing a necklace, watch, or belt of some sort, not to mention whips and chains if they are Dark wizards.

And of course, don't forget lasers. I bet they can do just about anything, and considering you can get laser pointers for less than ten dollars on Amazon, I'd carry one around with me just in case I smelled something funny.