finding out about gates before renting

[HOUSE_D]

Ok maybe I'm just missing this (most likely the reason)  but how can I tell what kind of rating a route will be without actually renting the gate.  Like I rented a gate at a airport and went to start a route from my base and it said it would be poor.  Is there a way to tell so I won't have to lose the rental fee first? 

[dktc]

Number of passengers the airport has is the indicator.

[pck]

Like dktc said, the passengers of the airport is the indicator, but it sometimes varies.
For example:

Your home base is A. It's gate value is 50,000 euros per gate. If you fly to Airport B, that has a gate value of 50,000 euros per gate, then you will get a poor profit, sometimes, even a loss.
BUT if A's gate value is 200-300,000 euros per gate, and you fly to B, which is still 50,000 euros per gate, you will get a higher profit, and it will probably be rated as medium.

This is from my experience (2 years+ experience, that is), so please correct me if I made a mistake

[CHR]

If you base in a sufficiently large airport (like Heathrow), you can fly to any airport profitably. It is the number of passengers in both your hub and the arrival airport that determines the amount of profit that can be made from a route.