Can recommend one of these inexpensive
Refractometers. Once calibrated with pure water they will measure a sample droplet against a scale of freezing point or SG of battery electrolyte to give charge condition.
Most useful I find to see what frost protection you have from coolant.
Yes, I've got one of these and jolly handy it is too. I use 50/50 blue glycol and water in my Mulfab ally radiator, it works fine, gives sufficient frost protection for my needs and I change it every few years. TBH I can't see the point of having anything fancy in there, it's all very well for the owners of cars which cover high mileages or which need to minimise their servicing costs but neither of those are relevant to me.
