Cheap newts of f8 or slower are very likely to be spherical as it doesnt matter as much as it does for faster newts.
There is a Synta made 130mm f7 newt which is known to be spherical and I would avoid that one. Cheap unknown brands I would avoid like the plague unless given the opportunity to test under the stars.
Apart from researching online, the only other practical way would be a star test.
