How do you know if you need an accent on 'a' or not?

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.