You really need to get hold of a water pressure guage, check out your local plumbing supplies, or think laterally and buy some cheap gadget from the likes of B&Q that happens to have a pressure guage on it!
If you are the clever type then you may be able to work it out by measuring the volume of water produced in a given time through a half inch hose
Seriously though, you can get a fair idea whether or not you have a decent pressure by simply using your hosepipe in the garden, if it fairly blasts out of the attachments you know you will have reasonable pressure.
But even if your pressure is low, you can get booster pumps for R/O systems that will give you the pressure you need.
Most of the systems on sale come with a water pressure guage as part of the package, but check first!
Best of luck!
Ian