Easy, you just guess the wrong pannel first every time, and then find out when the next page gives you the context.

Have the same question, but hazarding a guess - the rule of thumb would be using the bottom of the cells as guide?

I really am new to manga, but what I have noticed so far is that there are subtle flow queues in the art itself. For example, there might be a very thin line between those three cell and a thicker border between them and the larger cell to the left.

You tend to finish the right to left line to the end of the page before moving down any. So if 2 panels are in the same "line" like 2 and 3 are, then you finish out that line before moving down. In a case where one panel is larger and spans multiple lines it'll generally be laid out to belong to it's lowest line if it's on the left side of the page, or the highest line on the right side of the page. So since 12 covers both the line with 9, and the one with 10 and 11 on it you'll generally read it as belonging to that lower line and it'll be 12.