Scope

How to differentiate payment succeeded events for offline payment from that for an online payment?
Is there an event for a successful offline payment?

Solution
There is no separate event to identify offline payments against an invoice. You can make use of the "Payment Succeeded" event for this and identify if the payment was offline by making use of the “payment method” and “Gateway” parameters in the event content.

The value for the Gateway parameter is "not applicable" and that for the payment method parameter is the offline payment method chosen to record payment (like cash, bank transfer, check, Others, etc) 

Related articles and Documentation
Offline Payments Chargebee