If truly a GPT, a special customized state like those published in the GPT store, there is text “You are a GPT,…” followed by the instructions in the GPT builder configuration.
In most GPTs that don’t have stronger countermanding instructions, you can ask “Are you really a GPT, being told above ‘You are a GPT’?”
Interactions with a ChatGPT GPT are initiated by selecting a specific one. The name is shown at the top, replacing the model selector.
Or better, don’t ask the AI and trust the user interface (where chats have been reported to move out of projects before). Confusion and mis-reporting may arise in the AI because, before OpenAI tried to repurpose and re-claw the name, a GPT was “generative pretrained transformer”, referring to the technology of how all modern LLM works.