Common ethylene glycol is the antifreeze agent in most, but not all automotive antifreeze solutions. Common mix ration is 50/50 with water, and additive packages are included which commonly include dyes and more recently a taste agent to make the product taste bitter overcoming the normally sweet taste which has been a factor in the poisoning of children and pets.
At common temperatures the vapor pressure of water is far higher (300x) than the vapor pressure of ethylene glycol. For example, at 20C, water is 17.53 mmHg, and ethylene glycol (also called MEG) is 0.06 mmHg.
At higher temperatures that spread gets wider. Bottom line, an open container of 50/50 ethylene glycol and water, will result in most of the water evaporating first. If heated, at normal atmospheric pressure, then the water will evaporate even faster.
In older cars, tractors and other internal combustion engines, it is a common but not universal practice to run the cooling system unpressurized so as to reduce the stress on aging radiators and hoses. Overtime, with water vapor being released from a hot system, the mixture in the cooling system will become more concentrated. This will lower the thermal transfer properties a bit, so maintaining a mix mix near 40/60 to 50/50 of ethylene glycol and water will maintain a good thermal transfer rate.
Two common methods for measurement of the concentration are hydrometer (liquid density), or optically by use of a refractometer. In the US, presently, hydrometers are sold at car parts stores for under $10. The refractometer is a more accurate instrument, and can be used for battery electrolyte specific gravity estimations also is is commonly found for about $22.