What's actually included is the single most useful question — meals, guided excursions, entrance fees, transfers, and drinks all vary between operators, and two cruises at similar headline prices can include very different things once you look closely. See How Much Does a Nile Cruise Cost? for how much this affects the final number.
Ask for the ship by name
Which specific ship, and which cabin category, matters more than it might seem — ship size, age, and amenities vary widely even within the same general price bracket, so asking for the ship's name (not just "a 5-star vessel") lets you look it up and know exactly what you're booking.
Excursions and cancellation policy
Ask how excursions are structured — private guide versus a shared group, whether entrance fees to every site are included or only some — and understand the cancellation and rebooking policy before you pay, since river levels and scheduling occasionally shift an itinerary.