layers = self.iface.mapCanvas().layers()

found = false
for feature in layer.getFeatures():
    try:
        feature.attribute("fieldName") # Throws exception if it does not exist
        found = true
    except KeyError as e:
        # all features have hte same fields, so if it doesn't exists in the first feature, it won't exist in the others
        break
    finally:
        # all features have the same fields, so only need to check the first
        break

field_name = "my_field"
layer = iface.activeLayer()

field_index = layer.fields().indexFromName(field_name)

if field_index == -1:
    print("The field {} does not exist in layer {}!".format(field_name, layer.name()))
else:
    print("The field {} exists in layer {}!".format(field_name, layer.name()))