You cannot read a relay's routing table. You can send it a prompt whose answer only one model gives, and see what comes back.
The method
Pick a discriminating fact — a detail that frontier models know differently, or a style cue that is distinctive. Send the same prompt to the provider directly and through the relay. Compare the answer, not the marketing page.
Watch for false positives from injected system prompts that change tone without changing the model. See model authenticity and relays explained.