I normally fill up the port with standard fuel. The valves need springs to seal, although just pushing them in with a hand may also be enough if they are lapped well. If they are bad, they start to leak almost instantly. If I do not see any signs of fuel in the combustion chamber after some 30 seconds, it was always good enough for me (in this case, it probably evaporates faster than it leaks). On most bikes, especially if they were not running for a while, the fuel will just flow through the closed valve like nothing.
I think the bigger the valves are, the harder it is to seal them (need stronger springs). The valve seat angle is very important as well. The beauty of Ducatis desmodromics is that (apart from high rpm capabilities) there is almost no resistance when you turn the crankshaft (no actual valve springs), so far less losses there...