A simple way to check if a structure is level is to get a length of clear 10 or 12 feet long or so. With a helper, pour water into the tubing so that it is filled enough so that about half a foot or so is empty at each end of the tubing. You can add some food coloring to the water so that it will be easier to see.

Hold one end of the tubing so that the water level is at the same height you want your baseboards or height measurement to be. Your helper should hold the tubing at about the same height farther down at the potential baseboard’s height location. The water level at each end of the OPEN tubing will always be the same, so whatever height your desired water line is at one end of the tubing, the opposite end will be exactly at the same height. Mark each desired height location with a pencil. Move down a section and, using the same technique, raise the tubing again so that the tubing’s water level matches the pencil mark you just made. The tubing’s opposite end water level will match, so mark that location as well. Keep ‘leap frogging’ in this manner down one side of the room until you’ve made as many height marks you’ll think you will need for that side.

Once one side has been measured/completed, use the same technique when starting the opposite side by ‘bridging’ the tubing from the end of the first side you just did over to the opposite side. Make your mark on the second wall and then work your way down the second side as you did the first.

Note that the tubing length can be as long as you want though the longer, the better. Even a tube that is 100 feet long will produce the same result providing there are no air pockets in the piping.

OR

You can go out a buy a digital (laser) level for about twenty bucks.

Hope this helps,

Geof from the US