Basically you have 2 options:
- Use EntityQuery on the RegularResourceLink with an appropriate condition on the Resource field.
- Use WorkItemQuery specifying UserIds.
Option 1 is available in all flavors of the API (SOAP, REST and CZQL).
For similar 'how to's you may want to consult the Clarizen data model (http://usermanual.clarizen.com/15-4-3-1-data-model).
The 'Settings -> Configure' page in Clarizen lists all Clarizen entities with API field names (unfortunately the API entity names are not present. You may use the various Metadata API calls to get detailed information).
Hope this helps,