halfords do an anitifereeze 'pen' tester...about £2.......a glass tube with 3 plastic ball's inside,take a sample by suction and where the plastic ball's are positioned in the tube tells you how strong the antifreeze solution is(instuctions are on the side of tube)
As David21 says - you can buy inexpensive kits to test the strength of the anti freeze.
If you want a quick test to see if your anti freeze is strong enough, without having to buy anything, follow the instructions on this link (about half way down the page):
http://www.askthemechanic.co.uk/maintenance/check-levels.html
Hi Mr Gizmonster, thanks for the link, interesting reading. Now to hunt the kitchen drawer for a straw. Or maybe it is an excuse to go via a MacDonalds to purchase a milk shake after visiting the hospital. A milk shake is my only take-away I purchase.