The bricks should all have text on them (maybe a sticker, maybe molded into the plastic) giving an output voltage and current rating.

The output voltage should all be 5V (the USB spec). If you want to travel with just one brick, pick the one with the highest current rating and it should work fine for all your devices

(YMMV of course, especially if some devices don't really follow the USB spec. As always, test at home before hitting the road. )