Underneath each invoice in Billit, you will see a history of actions and messages.
If you see a line with an LHDN symbol this means the invoice was sent to the tax authorities. If the description mentions a number as well, this indicates that LHDN has successfully received your invoice.
This works the same way for sending via Peppol. If you see a line with a Peppol symbol, your invoice was sent to your customer. A number filled in under the description means that your customer also correctly received your e-invoice.