This article details several tips and techniques on how to guess what's happening (Or what's going to happen) in the series before anyone else does. Note that these guides can be applied to other book series as well, but I wrote them with the warriors series in mind. Enjoy!
1. Pay Attention to "Unimportant" Details
Many times, the books will mention many seemingly unimportant details which do not appear to contribute to the story. However, many of these seemingly unimportant details are actually clues to major parts of the future plot line. The key to finding these details is knowing the difference between a regular detail and a suspicious one. For example "The forest was beginning to recover" is a somewhat ordinary detail, but one like: "He noticed that she was looking proudly at... and wondered why..." or something similar to this that seems unimportant may very well have an important role in the future plotline.
2. Beware of Distractions
One thing I have noticed is that the books will frequently mention an "Unimportant" detail (See Above) and then immediately attempt to distract the reader from thinking about that detail for very long. Watch out for phrases like "But there was no time to think about that now..." or "He decided it wasn't important..." that immediately follow a suspicious detail. One example would be Omen of the Stars Book 1 Spoiler.
3. Pelt Colors Matter
When you are trying to determine a cat's parents, pelt colors can make a difference. While this is not always true, most often cats will have a pelt color similar to that of their parents.
4. Pay Attention to the Prophecies
Prophecies and omens usually contain many clues on future events. Even if they are only mentioned once and don't seem to be major, they can have great meaning in the future plotlines. For more information on this, see my article on How to Interpret Prophecies and Omens.
5. Other Minor Tips
Here are some other minor tips. First of all, Erin Hunter stated in a chat that you should pay special attention to what Rock says in the 3rd and 4th series. He knows a lot more than you may think. Another tip is that names can also be clues as to the future or past of a particular cat. For example: Power of Three Spoiler. While this can not be used as solid evidence of future plot lines, it can be used to supplement other clues and come to a conclusion.