How to Calculate the Pressure Difference

Could be anything from atmospheric pressure on up, depending on what you're doing with your condensate. Why dont you elaborate.

Measure it.

The pressure upstream of the steam trap is whatever the boiler pressure is, minus (relatively small) piping pressure loss between. The pressure downstream of the steam trap is whatever the condensing pressure is, plus (relatively small) piping pressure loss between.

Using pipe pressure drop formulas for various assumed flows, these two pressure drops can be calculated and graphed. (More flow, greater pressure drops in the piping, but less available PD through the steam trap). If you have enough data on the steam trap, it may tell you what the flow through the steam trap is at various pressure drops. (More pressure drop across the steam trap, more flow.) The two curves can be plotted. The point of intersection will then give the PD through the steam trap, together with the respective piping pressure drops unstream and downstream.

Much easier to put gauges on each side, if access valves are available.