How can you be sure you weren't placebo'ing yourself into feeling it?

To my knowledge creatine has no significant effects until your levels rise after, say, a week of taking daily

I've felt dehydrated while I'm in loading phase, it's very noticeable from that perspective. But the mental effects only come after at least a week (for me: longer attention span, less impacted by poor sleep).

Well.. I can't, nobody can. Perhaps it was a placebo effect or perhaps I had low levels of creatine and felt the difference. I often felt better right after taking it though, not consistently but often.

Did you inject or smoke it in order to feel "the mental effect just 2 minutes later"? Taken orally it can only be placebo effect.