In order to obtain the widget's method signature you can use its meta object information. For example, below is the Python script (Squish test script) that prints out widget's all public slots (method) signatures:
widget = waitForObject(':Test Widget')
metaObject = widget.metaObject();
methodCount = metaObject.methodCount()
for x in range(0, methodCount):
method = metaObject.method(x)
test.log(method.signature())