For fees paid for by tenants... Did you know, Massachusetts regulations require that real estate brokers and salespersons provide prospective tenants with a written fee disclosure if they are charging the tenant?  Unless otherwise agreed, this written disclosure does not create an agency relationship.  Even if the prospective tenant chooses not to sign the form, you should present a copy to the tenant and keep the original in your file.  You can obtain a copy of the Massachusetts Association of REALTORS form, for free, by going to their website.  If you have not yet registered for free forms, do that first.  If already registered, go here.

For fees paid for by the landlord... that disclosure and agreement is typically part of the agreement to lease the property.  MAR also has an Exclusive Right to Lease form that can help you establish fees you expect from the landlord, which you can access with the above links.