How do you debug custom validators from the panel?

I’m trying to write some custom validators for my panel and would like some direction on what is used to test them. I tried echo, but that doesn’t seem to work in the panel. Right now I have just a simple proof of concept file.

v::$validators['tst'] = function($value, $mynum) {
	echo("mynum", $mynum);
	return true;

Any direction you point me in for debugging would be appreciated.