Antifreeze tester from local motor factors usually 'bout a fiver. Take expasion Tank cap off, draw up some antifreeze and see where the float settles and read off scale & jobs a good 'un! Tend to check mine early October every year. 5 and a half years on its still showing mines good down to -29 degrees C
Google Maypole Antifreeze Tester... that's what I use