In Dynamics 365, when a plugin is executed, we can determine the query type that caused the plugin to fire through the IPluginExecutionContext in Microsoft.Xrm.Sdk.
In the example below, if we run create a simple plugin that runs on RetrieveMultiple, and display Active Accounts in the Dynamics 365 user interface, our plugin execution context will contain in the InputParameters a key for Query. The value of the key shows it is a FetchExpression query and the Fetch XML:
If we run the retreive multiple through other methods, such as using QueryExpression, this will be reflected in the input parameters. The options available for Microsoft.Xrm.Sdk.Query: