How to know where you are in the form

Here's the 12 Bar Blues Form... one bar for each chord symbol (roman numerals):

I I I I
IV IV I I
V IV I V

The only place where four bars of the same chord happens is in the first four bars (first line). So, if you don't hear any movement to another chord from the band for more than two bars, you know you're somewhere in the first part of the form.

In the second line, the IV Chord lasts for two bars and then the I Chord for two bars. If you hear two bars and then a chord change, then you're somewhere between bars 5 and 9.

In the third line each chord lasts for one bar. If you hear a chord change twice over a two-bar period of time then you're somewhere in Bars 9 through 12.

Simply put, the harmonic pacing (how fast at which chord changes) doubles for each four-bar line of the 12 Bar Blues.