BlockPos pos; //входные данные
if (block instanceof IFluidBlock)
{
if (((IFluidBlock) world.getBlockState(pos).getBlock()).getFluid().isGaseous())
{
return "gas";
}
else
{
return "liquid";
}
}
else if (block == Blocks.WATER || block == Blocks.LAVA)
{
return "liquid";
}
else
{
return "not fluid";
}