Unusually high electric bills, uncomfortable living spaces in the summer and ice formation on the outside handler during the summer months are all signs that the unit needs more refrigerant. Maintaining the air conditioning system regularly minimizes the leaks that lead to low refrigerant.
When an air conditioning system is low on refrigerant, it has to work harder to achieve the desired temperatures. In that case, it uses more electricity, leading to higher bills for the homeowner. Comparing electricity usage to similar months from prior years can help to determine whether the unit is working harder.
If the furnaces have been changed recently but the air conditioner is still taking what seems like an inordinately long time to cool the house, then low refrigerant may be the problem. Without enough refrigerant, the system cannot draw as much heat from the air in the house as it would otherwise.
Low refrigerant levels can also make ice form on the outside of the exterior air conditioner unit. It's one thing to see ice on it after a night with sleet or snow, but seeing ice on the unit during the summer months is a definite warning sign that something is wrong, and low refrigerant is a possible cause.