jackiedada's suggestion of a free ExpertFlyer subscription can give you a good idea of available seat maps.

Also, if you are purchasing a ticket with the US as part of the itinerary, you have a 24-hour cool-off period in which you can cancel your booking with no penalty (DoT regulations) - you can buy your ticket, check the seat map and then cancel if you can't find seats together.

Anecdotally, if your fare is cheap, there will be seats available to sit together as loads are low. If you are buying an expensive fare, it's more likely the flight is more full, but if you are buying an expensive fare, it suggests that your requirement to travel is more pressing than the seating together requirement...! Also, more expensive fares have lower (if any) cancellation penalties.