There is no way to guarantee either. Aircraft parking assignments are planned in advance, but subject to change without prior notice due to various reasons, such as the previous flight using the gate being delayed.
You can decrease the risk of a bus gate (stairs) by choosing flights that come and go during the daytime, early evening, as they do their best to park turn around flights at the gate for more efficient loading and unloading. Flights that arrive late or depart very early may use aircraft that park overnight, which would be more likely to use a bus gate to keep jet bridges open for turn arounds.
But if the flight uses a bus gate or stairs, then all airlines and airports have means to board passengers with mobility issues, ranging from simply carrying a specially designed wheelchair up the stairs, to using a lift truck to move the passenger and wheelchair up to one of the service doors.