Fee structures split roughly into two worlds. For rentals, agents typically charge somewhere between half a month and two months of rent, and who pays varies by country: in some places the landlord pays, in others the tenant, and in some it is split, so establish this on day one before viewing anything. For purchases, commission is usually a percentage of the sale price, sometimes paid by the seller, sometimes shared, occasionally by the buyer.
Exclusive representation agreements are common in purchase searches and less so for ordinary rentals. Before signing one, check the duration, whether you owe commission if you find a property yourself, the cancellation terms, and exactly which services are included. A short exclusivity period with a good agent can work in your favour; an open-ended one rarely does.
Get every fee in writing before engaging, ask what happens if a deal falls through, and never pay significant sums to personal bank accounts. Norms differ a lot by country, so ask locals what is standard.