When I was diagnosed, my doctor induced a hypo so that I & my family could recognise the symptoms. That was loads of fun...
There's a huge range of symptoms you can have, not all my hypos are the same. Sometimes I get shaky and weak and confused, sometimes I get cold sweats, sometimes a headache, sometimes I go white, and sometimes I just get unbelievably sleepy. Which is not a helpful one, as extreme sleepiness is also one of my symptoms of high blood sugar, and sometimes just a symptom of incredible boredom.
So my rule is that I test whenever I feel wrong. If I feel ill or a bit odd, I find it's easiest just to test, that way you can either fix your blood sugar or eliminate it as a cause of feeling weird. Just keep your test kit with you, stock up on strips (if I'm actually running low, I can guarantee that's when the GP will mess up my prescription and I'll be stuck for a weekend trying to survive on 2 strips), and always keep your lucozade with you.
Also pick an easily accessible emergency sugar. I use lucozade cos it seems to work fastest, although it is disgusting. But I can open the bottle easily, which is important when you've gone all floppy and your brain has left the building. Don't go down the same route as my nan - Nearly passed out with a level of less than 2, struggling to open a tin of pineapple with a knackered tin opener. Doh!