Do you have an outside tap?
If so then 2 approximate measures are either to see how far the hose flow goes in the air like you said ( my guess would be at least a metre or two above the highest point you want to reach) or, as my guy quoting did, whether you can stop the flow of the tap easily with your thumb.
Bear in mind that you will need to account for your internal pipework up to the point of inlet to the system so the pressure at the mains inlet could be higher than you'll get and the garden tap lower (especially if you are taking this from a self-drilling tap conversion kit in which case what's the pressure from the highest tap in the house?)
As has been said before, though, don't sign up to anything without getting the contractor to test the flow pressure.