Speak to the dog every time you go past it. Say hallo, etc...
Don't do anything more than that though until you've had a chance to speak to the owners and if they tell you the dog is friendly, then they can introduce you to the dog properly.
Some dogs can look aggressive, even when they are friendly. (my brother's German Shepherd is a good example of that, all he wants to do is play, but he just looks aggressive because he gets soo wound up)
And other's can look friendly right up to the point when they attack. (a Border Collie I meet when out walking looks really friendly, tail walking, etc... till he gets within 2 feet of another dog, then he goes for them, he's friendly with people though)
So it's always best to play it safe until you know for sure what temperament any particular dog has.