There are pressure testers out there but I test the water itself. I have a electronic TDS meter that will tell me what the PPM of the water is. If it reads less then 3-5ppm you have enough water pressure. If its a brand new Ro/di unit you should read 0ppm.
I have the 3rd unit down http://www.buckeyefieldsupply.com/showproducts.asp?Category=207&Sub=149
As a side note. I buy all my supplies from this guy as well. Replacement resin for DI unit and all the connectors I used to install it. It is listed under fittings on his website. It was much easier then finding the parts at the local home depot. And his shipping and delivery times are top notch.
http://www.buckeyefieldsupply.com/