Currently, it seems the only way to know is either
counting/remembering your turrets on the screen
looking at this tiny number in the bottom left https://i.imgur.com/DbkQhnJ.png
It'd be nice if it was easier, at a glance/peripheral vision, to tell if I'm out of turrets, so I don't end up mashing the Q button uselessly.
Maybe an idea is the turret icons are grayed out when you can't afford them?
Definitely open to other ideas, too.