I agree with our canadian friend here. When approaching a controlled aerodrome, ATC are supposed to provide clearances such as "cleared left downwind for rwy xx". That way, you always know. If this information is not provided, you can always ask ATC which pattern is in use "please confirm left hand circuit for rwy xx".
When arriving at an uncontrolled aerodrome, the standard is to pass overhead (and report over the radio!) and join a right-hand circuit starting at mid-downwind.
I might be wrong on this, but these are the procedures as I remeber them. And for uncontrolled aerodromes, allways report intentions in blind broadcast, you never know, someone might be up there with you!
good luck/lns