If I want to say, "you look tired", 疲れそう doesn't work, as that is "looks like I/you will get tired", but is 疲れたそう a thing? feels wrong...
Is it possible to use ~そう past tense? past negative? Is there a better way to say this?
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Yes, 疲れそう means "It looks tiresome" rather than "You look tired".
To say "You look tired", you can say 疲れていそう or 疲れてそう using the subsidiary verb いる. Other ways to say similar things are 「お疲れですか」, 「疲れているようですね」, 「疲れて(い)るみたいですね」, and 「疲れてる?」
疲れたそう(だ) means something like "They say he/she got tired," because this そう follows the dictionary form of the auxiliary た. For the conjugation rules, see Problems with そうです in this sentence
EDIT: So, the past tense is not at all necessary to say "You look tired." To be clear, this "(look) tired" is the passive form, not the past form, of the transitive verb "tire." If you need to say "You looked tired yesterday", then you can simply turn them into the past tense. The past form of ～そう(だ/です) is ～そうだった/～そうでした. 「昨日は疲れて(い)そうだったね」「昨日はお疲れでしたか」「昨日は疲れて(い)るみたいでしたね」「昨日、疲れてた?」