#1 clue that it is a destination wedding would be that nobody lives there in either family, nor does the happy couple, or the place is a popular vacation destination - whether known for weddings, cruises, movie characters, gambling, or the production of some popular comestible (cheese, wine, peaches, oranges, apples, etc.).
So - Las Vegas, Disney (followed by any one of the locations), a cruise ship - no matter whether headed to the Bahamas or whale watching, a vineyard for a wine tasting afterward, or any other event like that would be a destination wedding (possibly even if some members of the extended family or friends lived or worked there - as long as the happy couple & their parents don't live there).
#2 sign it's a destination wedding would be that there is at least overnight stay in a hotel because it is too far to drive - airplane travel would also indicate that it is most likely a destination wedding. If family members live there and have enough space in their homes for at least MOST of the bridal party, etc. to stay there...well, it can still be a destination wedding for the other side of the family and anyone who is having to take three days of vacation to attend might be excused for mentioning that it is a "destination wedding" as far as their budget is concerned.