The filament will draw considerably more current in air than in a vacuum, because the air molecules help carry away the heat, keeping the temperature (and consequently the resistance) of the filament from rising as much as it would if insulated by a vacuum. This is the principle behind the Pirani vacuum gauge:
http://en.wikipedia.org/wiki/Pirani_gauge
The best way to do this test (without risk of burning the filament out if indeed the tube is a leaker) would be to use a constant current power supply, set for just over the rated filament current (1.8A for the 15GP22). Apply power, and allow several minutes for warmup (with current limited, the filament will be very slow to come up to temperature). Then measure the voltage drop across the filament. It should be ~ 6.3V. If considerably lower, the tube has a very appreciable gas content.
Note that this simple test will not detect lower levels of gas that could impede tube operation. But it is a simple way to identify a tube with a gross leak.