Hi Steve
You would use the Method "getLeadChanges" and for the Activity Type: <xs:enumeration value="InterestingMoment"/>
See the SOAP API Doc here (Page 20) : https://community.marketo.com/resource/MarketoSOAPAPI
And WSDL (https://community.marketo.com/resource/MarketoWSDL)
Rgs
Damien