How do you check your undertone in daylight?

I've been trying to work out my undertone from phone photos, but the answer seems to change with the room. With my bedroom lights on, my skin appears yellow; near a window it looks much cooler. I would like a method I can repeat instead of trusting one flattering photo. My next step may be pictures beside an open window, with the same camera and no beauty filter. That should at least keep the lighting more consistent. Does putting a plain white card in the frame help spot white balance shifts? I don't know if bare skin is more useful than the foundation I usually wear. For a separate appearance check, I found color analysis and thought it might help me frame the comparison without treating a screen result as proof. I would rather compare several signs than decide from a single selfie. What would you keep constant when repeating an undertone check? I can also test colors against my face in person rather than relying only on the phone. A second check on another day might expose an accidental lighting effect. I'd appreciate practical steps, especially ways to catch a misleading camera result before buying anything based on it.