I've found that just pausing and saying something like "You probably don't want to be leaning like that, or else I could toss you" and then perhaps lightly indicating the start of a reversal, without actually going all out to exploit the 'error', is effective.
I've never seen an uke in my dojo throw a shite/nage/whatever term you care to use. It seems potentially quite dangerous to suddenly try to flatten the other person. What if they tense up, and your throw ends up injuring them? Better just to show them the error, and if they're curious, sure, you can actually do it. But I'd say not without asking first, both as a matter of courtesy (it could easily be construed as arrogance/"I'm better than you") and of safety.