Is there information if a given airline at a given airport is using a jetway or a bus to board?

The use of buses v's jet bridges will depend significantly upon the airport, and the type of plane.

Many airports do not use buses at all. eg, San Francisco airport does not use buses, only jet bridges.

Some airport only use buses for specific types of flights, often involving smaller planes. eg, Sydney airport uses jet bridges for all large planes, but will occasionally use buses for smaller jets/prop planes when they are parked too far from the terminal to allow people to safely walk across the tarmac.

Other airports can use buses for basically any flights. eg, Frankfurt airport uses a mixture of jet bridges and buses for all aircraft. The same flight might use a jet bridge one day, and a bus the next day, with no apparently pattern - even for flights as large as 747's.

There's not going to be a specific list available for which airports/flights use buses, but if you've got a specific airport in mind then you should be able to use Google to find reports of people saying they boarded by bus which will at least give you an idea if buses are or are not used at that airport.

Personally, having used buses to board at numerous airports, I really wouldn't consider it a sizable factor in choosing between one airline/airport and another. Unless you're traveling in First/Business class (in which case many airlines will run a separate bus or even use cars to get you to the plane), then the few minutes of being on a bus to get to the plane isn't going to be a significant part of sitting on an airplane for X hours during the flight.

I doubt there is an official resource where you can find this out.

Just from my own experience, I would say you can never be totally sure to get a jet bridge, the only available one may be broken!

In general, the bigger the plane, the higher the chances to board it through a jet bridge. You can usually find out the aircraft model when booking a flight.

Models for which I would expect a jet bridge:

Airbus A300, A310, A330, A340, A380, Boeing 747,757,767,777,787, MD11 L-1011,Ilyushin IL-86, IL-96

smaller ones where it is possible you wont get one:

Airbus A318, A319,A320,A321, Boeing 737, MD83 and many other small ones.

Wikipedia has a list of airliners by maximum takeoff weight.

It may also be possible that the airport does not have jet bridges.

For example, a year and a half ago I flew to Moscow from Zagreb with Aeroflot and since there is no jet bridge in Zagreb, we boarded the plane by bus and it was an Airbus A320. The flight back was boarded via jet bridge at Sheremetyevo, the same plane model. But the thing is that the gate number for our flight changed 5 hours before the departure and we would've used the bus if we boarded through the original gate that was on the ticket. Of course, Aeroflot employee phoned us to let us know about the change.

The only thing I can think of that you could try is to find out your gate number for a flight and then check airport plans to see whether that gate has a jet terminal or not.

P.S. I agree with Peter that it usually has to do with a plane model.