1 · Write your constraints down first
Before you see any vendor's screen, list your own hard and soft constraints from your collective agreement and the regulation that applies to you. A demo tuned to a generic scenario cannot fail against rules it was never shown, which is exactly why it always looks good.
2 · Ask for the mechanism, not the number
When a vendor states a percentage or a saving, the useful question is not whether you believe it. It is what was actually measured, on what sample, versus what was extrapolated or modelled. A vendor that cannot separate the two has not done the separation internally either.
3 · Test with your own data
A month that actually fought your current process, a real staffing shortfall, a real collision of two collective-agreement clauses, tells you more in one working session than a polished walkthrough of someone else's unit ever will.
4 · Ask what the tool refuses to do
A system that always says yes has no rules worth trusting. Ask to see a roster the tool rejected, and why, not only one it accepted. The refusal, and the reason attached to it, is where the actual rule engine shows itself.