grammar - verb た-form + そう, hearsay or observation?

Verb る-form + そう indicates hearsay.

帰るそうだ
'I hear he will go home.'
帰らないそうだ [Negative]

Verb masu stem (i.e. conjunctive form 連用形) + そう shows information obtained through the senses, observation:

帰りそうだ
'It looks like he will go home.'
帰らなさそう [Negative]

With this, how do you achieve past tense?

For "hearsay", do you say:

帰ったそうだ

' I heard he went home.'

Whilst for "observation", do you say:

帰りそうだった
'It looked like he went home.'

Answer

As Lukman pointed out, you got conjugations right. However, I am not sure if you interpreted 帰りそうだった accurately. In 帰りそうだった, the observation was made before the person went home. So in English, it is something like “It looked like he was going to go home.”