Formally, the number 1 cylinder is the cylinder that is "top-dead center to fire" when the timing marks on the crankshaft and camshaft gears are aligned.
To make it easier, the cylinder numbers are often stamped into the intake manifold, usually right next to the intake port, and also in a picture of all cylinders someplace on the manifold. It is also often on the EPA emissions sticker.
In an inline engine, number one is usually in the front, nearest the water pump.
4 or 6 cylinder straight blocks No. 1 is always the first one next to the pulleys. On a V block, if you look down at the top of the engine, No. 1 will be slightly forward. It can be on either side of the motor, but most likely on the drivers side.