I would go with your gut feeling after a fair amount of due diligence first... Check if the agents are experienced themselves, have they done any past developments themselves you can see? Do they invest themselves? Are there any bad reviews online? Check companies house records and if any of the directors have CCJs e.t.c.
They key is ask the right questions and don't just hand your money over. Transparency and honesty is everything so check the agent isn't being too "salesy" and gives you full answers to any of your questions. If they don't then be suspicious...
Of course anyone can ask on here for the community to give their opinion!