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

For jets, shoot for about a three degrees nose up pitch on final (for boeings, I believe airbuses come in with their noses a little higher)

1.3 Vso is used on jets as well. The problem with applying this formula to a jet is the wide variation between Vso(stall speed) at max gross landing weight and Vso at the min fuel weight (as much as a 50kias difference on some aircraft). The dynamics for a light aircraft are the same and therefore for Vso and obviously 1.3 times Vso will change with a fully loaded vs. a lightly loaded 172/Cherokee etc.. The change on a  light aircraft is so minute though, that most manufacturers tend to choose a fixed(safe) value and print that in the AOM.  This speed on a jet is referred to as Vref. Most jets are flown at Vref+10kts "ref plus ten" from the beginning of an approach, so that a nice stabilized approach can be maintained.
If you don't want to go through the trouble of researching "ref" speed formulas or tables, I  would just experiment with the aircraft in question.
Apply full flaps and extend the gear(landing configuration)and then slew the aircraft up to a nice safe altitude(5-10k feet). Establish a speed in the 170-200kias range and then apply the altitude and heading or wing level modes on the autopilot to allow the a/c to stabilize in both axes. Once the aircraft has stabilized, disable the autopilot and pull back lighty on the stick. At this speed the aircraft will increase pitch, speed will decrease and most aircraft won't stall. If the aircraft seems stable, continue this experiment but each time decrease the speed by ten knots. Eventually a speed will be reached where, when the AP is disengaged and light back pressure is applied, the aircraft will want to stall readily. This is the speed that I would assume to be at or near Vso for that particular weight. Multiply this speed times 1.3 and you should have a reliable Ref speed.
This will also give you experience in regard to what a crew must do when faced with a flight control or hydraulic flight control boost system failure.