You could surround your code using that external_id with try/catch and do the appropriate thing in the catch clause if that id does not exist. If you need to do that check prior to the actual use of it you could use the method get_object_reference from ir_model_data and surround the code with try/except, like this:

try:

self.pool.get('ir.model.data').get_object_reference(cr, uid, 'module_name', 'external_id')

except ValueError:

# do whatever