Question: How did Buddha know that he was enlightened?
Answer: The Buddha developed his mind so perfectly that he could survey the universe, after which he realised there are no beings who are just as enlightened — other than other Buddhas. It’s actually more relevant to ask this — How do WE know the Buddha is enlightened? Here is the answer: http://groups.yahoo.com/group/thedailyenlightenment-realisation/message/245 For ‘What if the Buddha is not enlightened?’, please see http://groups.yahoo.com/group/thedailyenlightenment-realisation/message/318
Question: How did the Buddha know his teachings are unsurpassed?
Answer: As above, he saw the whole universe and found no greater teacher espousing greater teachings. We too can survey history best we can to see if there are greater teachings by anyone else.
Question: How do I know if I’ve become a Buddha?
Answer: Some assume they are enlightened when they are not. To know if one is really enlightened like a Buddha, now that we have the historical Buddha’s teachings and know his conduct for reference, we can check it we are as true in thought, word and deed like him in terms of perfect compassion and wisdom. Actually, if we become Buddhas, there would be no need to check in this way, as we would be able to perceive other Buddhas directly — as our peers. The fact that we cannot do so readily means we are not yet Buddhas.