OK, so we know that going from one panel to the next (usually) means the passage of time as well, but how much? How do you establish the amount of time that a series of panels represents?
Usually, you don't, leaving it up to the reader to decide, but if that passage of time is important, the comics creator(s) may want to show it somehow. A caption might tell the reader that it is now "later", "soon", or "the next day", for instance – but that doesn't give him/her the exact amount of time that has passed. An easy way of doing that would be by somehow including a clock in at least the first and last panels of the sequence. If you want to emphasize the passage of time more, you can have a clock in each and every one of the panels. Since that is something that is usually not done in comics, it will take on a special meaning and emphasize to the reader just how important the passage of time is in that sequence.
But that is the objective measure of time. You may want to emphasize the impact of the passage of time on the characters in your comics story, and/or may prefer to keep your comic free from artificial gimmicks like the aforementioned clock. There are methods for that as well – and as usual, it helps if you're a good artist, as in this example from Marvel Fanfare (the artist may be Carl Potts, but I'm not certain; it's been a while since I read it). Here, the passage of time is indicated by the increasingly bored Spidey slumping more and more in his watch-keeping position up on a roof. We don't know exactly how much time has passed; we just know that by Spidey's standards, it's a lot.