Your eyes and your common sense should tell you a lot, and many folk keep year-to-year diaries of when there's been a late or early unseasonal frost.
If it's cloudy or windy, then it's not likely there'll be a frost, the wind keeps the air moving so things don't get too cold, and the clouds act as a blanket round the earth to keep the warm air in.
If there's no cloud cover, and the air is still, then there is a slight risk of a ground frost, it can be quite warm during the day, but without cloud, the temperature can plummet at night. Obviously you won't get a frost in the middle of July, as everything has been warmed by the sun (even if it's hidden from us mere mortals by cloud), but September/October time I keep a close eye on things outside if I've got any half-hardy perennials out. Another week or so and I won't be worried about frosts (this is Sheffield). The further north you go, the more likely a late frost.