Start with whether the profession is licensed where you are buying, because that splits the world in two. In licensed markets, agents hold a government registration number you can verify in an official registry, and unlicensed practice is illegal. In unlicensed or loosely regulated markets, membership of a national real estate association with a code of ethics and complaints procedure becomes your main quality signal instead.
Beyond credentials, look for: several years of traceable activity in the specific neighborhood, a physical office, transparent written terms, and willingness to say who pays their commission. For expat buyers, an agent experienced with foreign clients matters, since they will know the extra steps around permits and financing.
Red flags are universal: pressure to decide fast, requests for cash deposits before any contract, reluctance to put anything in writing, and claims of exclusive off-market bargains. Whatever the agent's charm, keep your own independent lawyer; the agent works for the deal, not for you.