To check whether your bike's earth is wired correctly you will need to run the dynamo, either on the bike or on the bench. The conventional earth for later A10's is positive so yours is probably wired correctly. You can, of course, 'flash' the dynamo to predetermine the earth polarity but my guess is that it is wired correctly and the ammeter is just wired incorrectly. Either way this is no big deal and I would advise you to get the bike running before worrying about the electrics.