Start with licensing where it exists. Many countries require agents to hold a licence or registration number, so ask for it and verify it against the official registry; in markets without licensing, membership in a recognised professional association is the next best signal, since members follow a code of conduct and complaint procedures.
Beyond credentials, look at behaviour. A credible agent is transparent about fees in writing, never pressures you to skip a contract review or pay cash deposits to personal accounts, shows you properties matching your brief rather than only their own listings, and is comfortable with you bringing an independent lawyer. Online reviews help, but weigh recent, detailed ones over star counts.
Word of mouth in expat and NRI communities is genuinely valuable here, as newcomers are the classic target for rental scams. Red flags: demands for money before any viewing, prices well below market, and reluctance to meet at a physical office. When in doubt, walk away; there is always another agent.