Verner Panton cone chair...How do I know the manufacturer?  

I'll be honest
I don't know. And I've had a couple of these.
I do know that there are different foot designs. The black plastic foot which is slender and follows the line of the leg, and the round nylon foot. I've seen both attributed to Plus-Linje, Fritz Hansen as well as Louis Poulsen, I've never seen a vintage example with a label or mark. What I do know is that it does not seem to effect the value regardless who the manufacturer was.
Sorry I'm not of more help.

thanks
I have the black slender feet. I initially wondered if it may be a fake due to lack of makers mark etc but the feet seem to be exactly the same as authentic examples being sold online...