As I well know from experience, calcium serum is inaccurate. Ionized calcium is better but the chvostek sign is my slam dunk that I'm low in calcium. In my case it means I need more vitamin D (needed for calcium absorption) as I eat enough calcium in my diet - cheese is my favourite food lol. Search on youtube for a demo: Positive Chvostek Sign showing Low Calcium Levels.
There are no devices available to test calcium levels at home. Calcium is part of the Comprehensive Metabolic Panel that your doctor can order whenever you have blood work, if you tell her/him your concerns...
Aside from that, make sure you eat plenty of calcium rich foods and/or take a supplement, as well as get plenty of vitamin D and magnesium, which are necessary for proper absorption of calcium.