Best way to check if a 20x25 floor is level before wood floor installation?

I'm about to lay down some wood flooring and I need to make sure the subfloor (3/4" plywood) is completely level. What's the best way to do this? I was thinking about laying a 7ft straight edge down every which way but there must be a better way. Any suggestions from the pros?

2 options

1) snap line.
2) laser level.

run a level line from wall to wall pull it tight if no gaps under line then floor is level. you can be a little out depending on what type of floor you are installing. 3/4, floating, etc. the striaght edge is fine too since the floor should be level within 8 ft for a floating floor. sand down any high spots.

level isn't as important as flat. some variance is acceptable. for level wall to wall try a water level and a mark on the wall in various places at the same height from the floor of course. some out of level gives a house a bit of charm. ps, i do like the marble thing.

For inside levels your straight edge would probably be the easiest.A longer one would be better still.

pull a tie line from corner to corner and measure with a 3/4 in gauge made out of wood and just slide it under the cord. make sure each end of line is 3/4 inches off the floor.

I would use a laser level, measure the distance from laser line against the wall to the floor several places on all four walls. If your don't have a laser level use string instead. Measure one distance in a corner then same distance in the opposite corner and run the string from corner to corner. Then if you have a line level (2 bucks from Lowes) place it on the string to see if its level. That would be an easy way to check it.

Throw a bunch of marbles on the floor and see which way they travel. Other than that, just continue on with the straightedge.