how to test water pressure coming out of pipes w/o spilling water
There are several factors that should be considered.


If you are on a public water supply or a well.
The size of the supply lines to the bathroom. (typically 1/2" or 3/4")
Length and height of run from main water source.


The pressure can be measured with a gauge that attaches to the water pipe or a faucet at the area you want to use. Typically a pressure of 50 to 70 psi is needed for your application. 

The second measurement is capacity of the feed line at the user point. Test this by running the water into a calibrated vessel at full open for a set period of time. 

Example: Full flow for one minute, then measure the amount of water. The result will be the gallons per minute. Compare this result to the amount of water the new fixtures need to operate properly.  If your supply gives you 5 gallons a minute and each shower head needs 1 gallon a minute, then you have ample supply. However, it you only have 3 gallons a minute flow and you need 4, then obviously the flow from the four shower heads is going to be too low. 

I certainly would do some testing and calculations before doing an expensive renovation as you describe. You also want to look at the hot water capacity of your system to be sure it can keep up with a larger draw of hot water.