I have a hard time believing anything anyone says without proof. So if they don't show, I don't consider what they say to have any merit. Also, I never show cards when requested (that's not to say I don't ever show. Just I don't show when requested.) Showing cards is to help my game, not yours. And if you ask to see, you must feel you can help your game by doing so. With that in mind, I never ask to see another players hand. Asking in itself can give away part of your own game, and could hurt you when you take time to consider plays in later hands (based on what the person told you, you could be swaying your decision on vague info.)
All that being said, I have to say, I'm really bad at reading other people and determining what they have. I simply play my own game. I either have the hand, and bet like I have it. Or I decide to bluff, and make an early decision on how much I'm will to chance on the bluff. It may hurt my game in the long run, but too many times have I guessed wrong on what my opponent had, and lost the hand.