How do I know if a customer was sent an automatic receipt when I received their payment?

[Coachkercher]

How can I tell if a customer was sent an automatic receipt when they completed a transaction with me? I want to avoid resending then a second receipt if they already got one, but I can't tell. 

[RHatch]

I don't think there is anything that says it sent and email.  But is is automatic when you are using invoices.