The old way to do it is with a very thin piece of thread and a helper. I'm assuming the frame is straight. With the chain off tie the string to the front part of the center chain ring. The idea is to pull the string taught back towards the freewheel/cassette so that the string lies right across the chain ring. At that point, the the string will line up with one of the cogs on the freewheel. If it lines up with the center cog on an odd numbered cogs freewheel/cass or between the two center cogs on an even numbered cogs fw/cass you are ok, If not you have to adjust.

What I do is tie the back end of the string to something heavy, but movable, like a car jackstand. Then I have my assistant watch the string in relation to the chainring as I use a mallet to nudge the bottom of jackstand sideways until my assitant tells me the string has just touched the back part of the chain ring.