The symptoms of hypos vary a bit from person to person.
The early signs of them will probably be a vagueness in responding to something that is asked but if not picked up on, a stiffness can develop affecting the way people stand, hold their arms etc followed by a paleness in face colour. If out walking, difficulty will develop as balance starts to be lost resembling in severe cases as 'being drunk'. Most diabetics feel very embarrassed over hypos and dont really appreciate being humiliated.
My advice to you is that with time you will be able to spot the early signs yourself and just be nice, calm but also firm and just say that you dont think your partner is looking right and that they need to do a bg test and if they dont because of arguing, then just make them a cup of tea and put 3 tsp sugar in it and with any luck, he wont know the difference between sweeteners and sugar.
Hope that helps you a bit but do talk with your partner and tell him not to feel embarrassed about hypos and to tell you that he needs to test and eat something.......