I was under the impression hätten meant "would have". But I just saw it in a sentence that contained "sie hätten," where the translation was "they had."
How can you tell when it's saying "they had" or "they would have"?
Do you mean this sentence?
"Sie sagten, sie hätten dieses Auto nicht geprüft."
If so, check that discussion page for various explanations. Short answer: If it's reported speech ("They said that they had...") you'd use Konjunktiv I, which in this case is identical to Konjunktiv II in spelling.
Hmm. Could be a typo then, because what you're saying would make sense with hatten instead of hätten.
One more thing occurred to me, which is that the subjunctive hätten here could be used for reported speech, if it was something like sie sagten, sie hätten... and then it could be "they said they had..." But that's total guesswork without the sentence.
This usage checks out on Reverso
I just did those exercises recently and it was definately 'hätten,' not 'hatten.' I was surprised too, but I have noticed some super-educated Germans I know, do use it to mean 'would have.'