Does anyone know if I can run at 250 watt HPS bulb on a 150W ballast? Or will it create a fire hazard?

Depending on what line voltage you use the answers are very different

120V :
250W lamp won't work on a standard 55V 150W ballast. It will glow on the ignitor output and try to strike, but won't get anywhere. The ignitor will be overloaded and will fail if left for long like that

230V :
250W lamp will start and warm up to partial brightness on a stanrard 150W ballast. The ballast will be somewhat overloaded and run overheated. If it is cooled exceptionally well and not heated by the lamp, it might be able to handle that. In the general case, it may lead eventually to ballast failure

Will I be able to at least test the bulb with the 120V 150W? I just need to make sure it works.

Will I be able to at least test the bulb with the 120V 150W? I just need to make sure it works.

As Ash said, it won't strike, so no. Always best to use the correct ballast for the lamp.

Ok thanks for your help tho. Take care