Yep, unless you are pretty much looking at it at the time it forms, you won't know until it's darned near on you. About the best you can do is watch the cells and convergent zones on radar and try and assess the risk. I get a kick outta watching these "Storm Chasers". What fool would drive into a cell at night knowing it is likely to produce a twister somewhere near you? Even these guys with their own portable doppler systems can't always make out the telltale pattern until they get a visual confirmation anyways. The best you get is the ability to predict when the conditions are good for one to form, but that is often just hit and miss, and not much better than when Uncle Jimmy would stick his head out the back door and look at the sky.