Hook it up to a bench power supply and measure the current demand for the +15 and - 15V rails. Next look as the physical size of the transformer. Go and search for transformers of that size with secondaries in the range 15-0-15 to 18-0-18 (guess for now) and note the the maximum secondary current, primary and secondary DC resistance ( or regulation if not available).
Now download Duncan's PSU designer tool (search for it) and plug your transformer data in to see if will work. Your goal is to ensure that the input to the regulators is not less than 18V at full load and lowest expected line voltage. On the other hand you don't want the voltage too much higher else you might overheat the regulators.