Lucknow has no shortage of cab operators, but service quality varies enormously once you leave the city limits. The checks that matter are unglamorous: is the quote written, is the vehicle commercially registered, and does the operator brief the driver about your itinerary before the trip starts?
Ask how waiting time is handled. A serious operator will state a free waiting window for airport pickups and a per-hour rate beyond it, instead of improvising a number on the day.
Look at how the fleet is described. Vague promises of a “luxury car” mean little; a clear vehicle class with seating and luggage capacity means the operator understands what you are actually buying.
Finally, judge the response. A considered reply that confirms route, vehicle, timing and inclusions is a much better signal of reliability than the fastest or cheapest answer.