When roaming in Local Break Out (LBO) the data stays within the VPLMN and the data has not to be forwarded to the SMF and UPF of the 5G core of the HPLMN. While this is attractive for the latency I'm wondering how the consumed data usage claimed by the VPLMN can be trusted. Is there some kind of interface reporting the data usage periodically or any other mechanism which can improve the trust or even remove the need for a trust relationship at all?
The usage data is trusted because it is trusted. From the perspective of the customer, the measurement of the service provider is inviolable. They can try to dispute it if their device shows a different amount of usage but the service provider's policy will take precedence and the customer may simply be offered some kind of accommodation as a service credit or similar placation.
The service provider has to trust the equipment vendor that the usage measurement is reasonably accurate and they have to trust usage data reported by other service providers when the client device roams outside the HPLMN (if the service contract specifies such ability).
Obviously the service provider equipment provides usage accounting for each IMEI and SIM in use on the network, that is a basic function of the service provider network platform. If those measurements are inaccurate, it would be easily revealed by any interested party who wanted to test. They could simply buy several test phones, a few matching pre-paid or post-paid accounts for comparison on different service networks and then exceed the included usage and see how accurate the service provider measurement is. Given that it is so easy to test, I see no reason to expect they are inaccurate.
I don't believe there would be some kind of third party measurement that could be more accurate than the measurement coming from the equipment that provides service to the end user devices.