[seL4] How to get a helper thread's TCB?

Hi guys, Currently I'm working on something that requires threads can be moving between domains. I can run two domains now (trying to run more) and I'm trying to understand the content of domain.cxx in sel4-test. Each thread has TCB which contains the domain id of it. But after running through *struct helper_thread* and relevant data structures, I'm curious about how to retrieve a helper thread's domain id, which could help me verify that they're indeed moved to another domain. Best regards, Kun Cheng