I'll give you two options for making a level of your own. Both are based on the simple fact that water is level, and will keep being level even going through a hose.
Level based on glass of water
The most rudimentary level you can have is a single container of water, and just watch the level of the water. This can be a glass bowl, or as simple as a single glass. If you combine this with a straight piece of wood, you have a rudimentary level.
This can give you an accuracy of centimeters or parts of an inch, if used carefully. And the combination of a 3' / 1m piece of wood and glass, can be used together to measure if different furniture are reasonably level. You could even use the water glass / glass bowl directly on the furniture to verify that it is reasonably level.
Added: A variation of this, requiring a little balance act, is to use two glasses one at each end of the plank. And then you can visually align this to get a sense on how unlevel it is.
Level based on bottles and a hose
A much better level requires a little more equipment: two clear bottles, a hose and some tape to join it all together. The base setup is like in the image below:
To make this you cut off the bottom of the two bottles, and join them to the hose using tape. Fill up from one end until you have water appearing in both of the bottles.
This arrangement will have the water stabilising at the same level. If you have a few friends helping you, you can measure distance from the water level to your floor and decide what is level.
Another option using this level is to use it close to the wall, and mark on the wall (i.e. using tape which can be removed afterwards) where it is level, and then measure out the floor at different points.
Or you could use a tripod and fasten one bottle, measure the distance down to the floor, and then move the other bottle around and measure the difference of distance to the floor on different locations.
Added: Extended usage example
First of as Zaaikort indicates, you can also use a (semi-)transparent hose without the bottles as the level. In my experience though, it is a little easier with the bottles.
One way to 'calibrate' the system is that you attach one bottle (or that hose end) to a chair (or stick) and make a mark on the bottle at the water level. Hold the other bottle next to it and make the mark at the other bottle as well. Make a note of the length down to the floor from the water level.
When you now move the loose bottle around in the room, you can measure the distance from the water level and down to the floor. If it is not the same as the original distance (from the chair/stick) to the floor, you are not level and you can subtract the two values to see how much it is off.