Yup, there certainly is a "float test" for the single-carb setup! "Throw it into the lake; if it floats retrieve it for further study."
The single-carb setup has a few deficiencies for our motors. The largest is that there is no provision whatsoever for manifold heat. The long unheated intake runners mean that the fuel will come out of suspension from the air stream, and wet the manifold walls down. This leads to an uneven mixture.
The single-carb setup tends to distribute fuel somewhat unevenly between the cylinders, too. This is probably at least in part due to the manifold wetting issues, above, but may also be due to the design of the manifold itself.
Finally, they are a bitch to tune for our cars. Not sure on the "whys" of that, but it's tough to get them set up decently for our cars at all!
Duals have none of the above problems, if they're working properly. They have short manifolds that sit directly over the (HOT!) cylinder heads, so everything gets nice and warm and the fuel stays in suspension.
--DD