water level as mentioned. I would use a laser level. I have a cheap black & decker one but I think its discontinued. There are a bunch of self leveling one out there that are cheap enough and very handy. They draw a level horizontal laser line.
When you have the line you can just move a yard stick or something around and measure down from the line in various spots. Make a mark on a door jamb where the line is and then move it to the next room and continue.
With this method you can find the lowest spot within the accuracy of your measuring tools.
Oh, but you really only need to be close. Sometimes convenience of installation comes into play as well. Also, if your water issue is below slab ground water then your lowest spot is the bottom of the sump bit. If its surface water you should address your grading, gutters downspouts, etc.
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