Why Does Tester Bulb Glow on Both Wires of Single Phase Diesel Generator Output?

Why bulb of tester glow when connected the out put of a dg one by one
Or
How to distinguish phase and neutral at the out put of a single phase diesel generator

Check the AC voltage between each of the outputs and ground (or chassis). You will probably find that both of the outputs are isolated from ground/chassis so there really is no phase/neutral.

If this is a 110V output on a 220V generator or a 220V output on a 440V generator then the low voltage outputs will probably be between a "neutral" and on the legs of the higher voltage output. This can also be checked with an AC voltmeter. The higher voltage outputs will likely be phase to phase (180 degree) with a neutral in the middle.

I believe there is a standard for phase/neutral on the twist lock connectors that are on most but the smallest generators. Even if there is no actual phase/neutral distinction on the generator output, if you are wiring outlet adapters you would maintain the conventional connections to the twist lock.

By all means, ask a qualified electrician if you are not sure, mistakes could create a fire or electrocution hazard.

Further to what Dean said, in a fixed installation, where the generator is hard wired into a distribution system, it is likely that there will be a point somewhere where the chassis(earth) and one of the two wires from the generator are tied together. This is normally at the main distribution panel,
cheers,
Richard