As long as you are using an ethylene glycol (not a propylene glycol) based antifreeze, a cheap hydrometer will work. It won't be very accurate (reading depends on fluid temperature) but it gives a good starting point.

If you are using a good refractometer for testing, you will get much more accurate results.

The real issue is that if you are worried about the effectiveness, it is usually because water has been added. In that case, even if the freezing point is sufficient, the additive concentration (the important things that protect cooling systems) will likely be low. The safest thing to do is to put new coolant in, at the correct ratio. As soon as it isn't -30 C out anymore.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue