I cannot remember if it's the alt/option key or the command/control key because I checking before I'm on my phone, But if you hold one of those down and click on the display you can switch between timecode, frames, or feet and frames.
While set to show Time Code, After Effects displays a "f" for frames, "s" for seconds, or "m" for minutes in the Time Ruler based on the current Timeline Zoom level and it's entirely automatic. There's no preference setting for "always show full time code in Time Ruler". (This could make a good feature request though.)
So, there's no way to force AE to show you "02:00f" if you've zoomed out far enough for it to show you "02s". Accordingly, there's no way to force AE to show you "02:00:00f" or "02:00s" if you've zoomed out far enough to see "02m". You have to zoom in for the "m" to become a "s" or for the "s" to become a "f".
I find it helpful to simply look at current time value in the Timeline panel or in the Comp panel which, as I'm sure you've already noticed, doesn't have a "f", "s", "m" because it's always displaying full time code (so, "0:00:02:00" when at "02s" or "0:02:00:00" when at "02m").