We are an agency, so read this knowing that. It is still worth writing, because the checks below are quick, they are public, and most people buying in Dubai skip them.

The five minute checks

The ORN. Every licensed agency has an Office Registration Number and it is verifiable. Ours is 28357. If a firm will not give you theirs, stop there.
The agent’s BRN. Individual agents are registered separately. Ask for the broker card and check the name matches the person in front of you.
A real address you can visit.
A landline, not only a mobile.

What actually separates good from bad

Do they tell you what is wrong with a property? The single best signal. An agent who has never mentioned a drawback is either inexperienced or selling.
Do they know the service charge? Not the community average, the figure for that building. It is the number that decides your return and it takes a phone call to get.
Will they say no to you? An agency that talks you out of a bad unit is worth ten that agree with everything.
Do they answer after the sale? Ask for a client who bought two years ago and call them.
Do they hold the keys? An agent without access cannot show you the property this week, whatever they say.

Things that sound important and are not

Office size, awards, the number of agents, and how many properties they list. A firm with ten thousand listings is not better placed to advise you on one apartment. Volume and judgement are different things.

What to ask before you sign anything

What is the total cost of this purchase, including every fee?
What is the service charge for this specific building, in writing?
What else completes nearby in the next twelve months?
What would you not buy in this community, and why?
Who else is marketing this property, and at what price?

If an agency cannot answer those five, the problem is not the property. Our own team, with their BRNs, is on the team page. Also useful: how to choose an agent.