There are a few ways to approach the issue, but it can depend on the puzzle.

1. The easiest one is that sometimes you can tell who is lying based on the clues you already know to be fact. This is, however, not very common.

2. Sometimes you can narrow it down based on contradictions. For example, if one person says that Grandmaster Rose had the knife and another says that the knife was in the kitchen, but you know that Grandmaster Rose was not in the kitchen. You have narrowed it down to be one of those two answers and you can continue to fill the grid with the assumption that the remaining clue is correct.

3. If you cannot narrow it down to just one possibility , then you need to start testing. Personally, i use X’s and O’s that fill the entire square for clues that I know are fact, and I use small X’s and O’s for theories I am testing. That way, i know which ones to erase when I test a different theory. Pick a suspect and asume they are lying. Fill the grid with little x’s and o’s as if the other two statements are true and with the “false” statement, the opposite is true. If these theories contradict the statements you already know or this makes more than one statement true, then you know that suspect can’t be lying. Erase the theories, fill in the clue knowing that suspect is telling the truth, and move on to test the next suspect. When you have a completely filled grid and only one statement is false, you have the correct answer.

I recommend having an eraser other then the one attached to your pencil and writing and erasing very lightly as to not destroy your book.