Give it the three things a price needs
A price exists only for exact dates, a number of guests and one place. "Hotels in Lisbon next month" gets you a range; "two adults, 14 to 17 November, central Lisbon" gets you totals.
Ask where the number comes from
A good answer says whether the figure came live from the hotel's own system or from a page it read earlier. Ask: "Is that the live total for my dates, with taxes and fees?" If it cannot say, treat the number as a guide, not a quote.
Ask for the total, not the nightly rate
Nightly rates hide cleaning fees, city taxes and service charges. Ask for the total for the whole stay, and what it includes.
Book on the hotel's own site
The safest booking is the one made directly with the hotel or its chain. Ask the assistant for the link to the hotel's own page for your dates, and check the total there before you pay.