I have an oil painting by an artist called D. Man.

I have no idea what it is worth, if anything.

Asian/Oriental narrow street market in bad weather. Could even.be a market in India.

Who is D. Man and is his work worth anything? Should I stipulate it on my insurance?

You cant insure if you don't know the value, unless you just name a value you want to be paid should anything happen to it. If you want it authenticated then you will have to take it to a local gallery.