If you have access to the database itself, the simplest way to verify is:
Create a context with the persistToEDM flag set to true => {"contextId":"PersistedContext","persistToEDM":"true","data":{"key1_name":"value1_data"}
Run an SQL query to search the database for this context => select * from cs_operation where context_id = 'PersistedContext'
If you cannot query the database directly, you can use ContextStoreQuery ReST interface to verify that the context has been written to the External Data Mart => http://clusterIP/services/ContextStoreQuery/edm/contexts/audit/{{contextId}}/?data=true