If you're not quite comfortable wearing a Wifi detecting Tshirt, then this $8 gadget might be the next best thing!
The nifty Wi-Fi Detecting Pen is able to sense wireless hot spots, and has four LEDs that light up like a bar to indicate the strength of the signal. It can inform you of Wi-Fi from up to 30 feet of a hot spot. It runs on two button cell batteries and yes, it even writes.
It beats busting out the laptop, finding a plug, setting it up, only to find out there's no wireless service and having to get up and set up over again. The only problem I see with these types of gadgets is that you're unable to detect whether or not they're security enabled. Having your device inform you there is webability (it's not a real word, but it should be) in the air is pointless if you can't make use of it.