Since GAE datastore does not have an equivalent to MAX
on an SQL database, it is indeed best to order by the column and retrieve the first entry. For example:
max_val = MyEntity.query().order(-MyEntity.my_numeric_field).get().my_numeric_field
See also this forum discussion.
Your second option would be to store the maximum value in some other entity and then update it (probably in a transaction) whenever you add one of your first entities.