The best bet is to watch your local weather forcast, or log on to your local news channel's website and look at their weather report. Most will give you multiple-day predictions, so you can look ahead and see what the lows are predicted to be. Or, you can go to www.weather.com
for a look ahead. Your goal is to bring your tropicals in when nights consistently stay below 50; if you're getting close to having frosts, then it's typically past when the trees should have been brought in. I don't know of any service that would email you frost warnings, though there are lots of apps that will send weather info to a smartphone.