You can see it in cross section(setup-cross section).
Also you can see it while generating gerber's(manufacturing-artwork), there will be a plot mode(either positive or negative)
It is always better to use positive artwork.It will be easier in the validations tools to work with positive artwork.
Positive artwork file size will be bigger than negative artwork but now a days with GB's of RAM we dont need to worry about the file size.
Also it will be easier for checking when we use positive artwork.
in my point of view negative artwork is best for power and ground plane because we can give accurate user defined thermal relief(flash).am i correct guys?