If you don't know what speed to approach at, is there a way to 'guesstimate' the right speed

Not sure if this applies to heavies, but for light aircraft, the recommendation( if no approach speed is specified) is to go with 1.3 Vso (stalling speed with gear and flaps down, usually the bottom of the white arc on the airspeed indicator). Makes sense, as Vso can also be described as the minimum speed for the landing configuration.
You may control how steep your approach is and how fast you descend, without changing this speed, by using power and pitch...