the best way to know you crit would be the crit chain skills opening up, this skills should also light up a little around there box.
you should also get a little sound when you crit, although I'm not sure this is an option or not.
Check in combat options -> Enable combat partial response indicators
hopefully that should help without the need of plugins.
if you want plugins for burg I'd suggest asking in the burglar section as I'm sure you'll get a better responce