Some centuries ago, when hell was frozen I made a summer internship at MEA
(Middle east Airlines AirLiban). One of my (many) questions is how to assess the potential of a line. The manager of route development told me that there were many indicators :
1- Number of visas delivered between the 2 countries.
2-Expatriate population living in the 2 countries
3-Foreign investments made by either of 2 countries (business travel , cargo)
4-Passengers connecting at intermediate airport to .
All these information are made available through info from embassies and consulates and do not forget that in Africa, Gulf and Middle East passengers are still requested to fill in a disembark card with all appropriate information ( country of residence for ex, purpose of travel etc...)
Hope I have been of some assistance to you