Is it the verb 'avoir' or the preposition 'à'?

To know which one to choose there is an easy question: can I say 'avait' instead of a/à? If the answer is yes, then it's the verb and it doesn't have an accent.

Exemple:

• Il a/à un chat. => Il avait un chat. 

As we can say 'avait', you don't need an accent. The answer is: Il a un chat.

• Viens a/à la maison demain. => Viens avait la maison demain. 

In this case you can't replace it by 'avait', therefore it's the prepositon 'à'. The answer is: Viens à la maison demain.