I don't know the exact method, but with fuel stoves, i.e. wood fired stoves for cooking, there used to be a method where you put a piece of white paper or some flour in the oven and you could tell the temperature by how how quickly and how brown the paper or flour went.

I Googled but the one promising link was disconnected. I remember seeing that in an old cookbook but I don't know if I still have it.

Helen