Yep... do these things that Pat suggests.Go meet them, meet their dog and make friends and then ask the people if their dog is "dog aggressive" If they say no, then have a very supervised "meet and greet". If that doesn't go well, then tell the people you walk your dogs every day and what do they suggest? Then carry mace.
Was it a "beware of dog" sign? At least they have given a "heads up".