Q: How do I find which fields in a Salesforce object are Compound fields?

A: Run this SOQL query:

SELECT Label, MasterLabel, Name FROM EntityParticle WHERE EntityDefinitionId = 'Opportunity' AND FieldDefinitionId IN (SELECT DurableId FROM FieldDefinition WHERE IsCompound = TRUE)

. . . change Opportunity to the API Name of the object you’re querying.

The query above is mighty useful when you’re querying the Bulk API.

The Bulk API can’t handle Compound fields.

Props to this StackExchange post!