Get a thermowell for you temperature probe (i.e. probe is in the beer) and then set your freezer/fridge to the exact temp you want fermentation happening. That way you can be sure.
Brewing since 1989 - BJCP National Rank
Conditioning: Burton Ale, Belgian Quad, Old Ale
On tap: Cider, Impy Cherry Stout, Irish Red Ale, Irish Red Lager, Big Irish Red Ale, Rye Dunkel Bock, German Easter Lager, Mexican Lager, Belgian Dubbel Chocolate Stout
Newly Bottled: Dubbel, Belgian Golden Strong, Belgian Easter Ale