How to test emails that reference custom object data using live records

Hi there,

We have had emails setup in our instance which reference custom object data (i.e., $TriggerObject.lastName). Normally when I test emails which use standard customer data, I just refence a record that contains this data and the email renders correctly.

I'm guessing that this is not how you test emails which use custom object data. I can see records which have the correct custom object data but I can't seem to get the email to render this data in the email when I reference these records in an email preview. 

I also found this piece of information and I followed these instructions but that didn't work either. "You can test your script using the “Send Sample Email” email action within the Marketo email designer. For the script to process correctly, you need to select an existing lead to impersonate in the Lead field. If you’re testing with a $TriggerObject, you can select the triggering object via the ‘Trigger’ param. This will use the data from the most recently updated object of that type as the $TriggerObject variable."

Does anybody know how I can test these emails to make sure they're working correctly?

Thanks in advance

Caleb