In most cases where the shift pattern or number of gears is unknown, one would toe down (counterclockwise on the shift shaft) multiple times until it doesn't feel like it's shifting anymore. On most setups, this is first, but in some cases will be neutral. Turn the output sprocket. If it will turn freely, it's neutral. If it engages the engine, it's first.
Please note that some trannies don't like to shift the whole range with nothing turning. Roll the output sprocket back and forth a few times while shifting to make sure it's going all the way.