I THINK THE ONLY WAY TO BE SURE IS TO GO ON TUESDAY NIGHT!
I was planning on filling up last night (get paid Friday) for around $1.389. I started getting that weird feeling b/c I was seeing alot of $1.589 all over town. Turns out, prices must've went up in the middle of the day Wednesday.
But like I said, I have a couple of stations that lag behind in price fluctuations. Turns out the Shell (I usually use BP) still had gas for $1.389. So I got 14.3 gal of Shell 87 octane for $20. Would've costed me $23 @ $1.589/gal!