Are you feeding from a real bottle with real gauges or one of those garbage walmart cans that let in as much air as refrigerant?
I hope that if you have a leak, you're following the law and fixing it properly before charging.
Doing it right, with gauges, will tell you that there is a static pressure that varies by temperature, that will indicate a two phase system. The low pressure switch will likely be much lower than that pressure in the summer time, likely by a factor of three. So I'd just let the gas pressure rise off a bottle and it will trip the switch.
But again, if you lost the refrigerant not through a proper recovery process, you should pressurize with nitrogen and a trace of refrigerant, sniff it and find the leak, fix it, recheck, the. Pull vacuum and refill properly.