Clarifying usage and definition of 怀疑 怀疑 means (1) to doubt, and (2) to suspect. This seems to lead to some confusing translations. For example:


  我怀疑这张百元钞票是假的


could have two very different translations. If I use "to doubt", then it would translate to


  I doubt this 100 yuan bill is fake


Meaning that you think it's real. On the other hand, if I use suspect, it would translate to


  I suspect this 100 yuan bill is fake


Meaning you think it's most likely fake. These are nearly complete opposite meanings! 

I don't believe my understanding of the word is wrong b/c I've heard it used in both contexts (though I may indeed be wrong), but how am I to know when definition is being applied?
First we will deal with the origin (English):  

doubt is, in a sense, questioning something
suspect is, on the other hands, distrusting something  

back to your example, you can identify the 2 sentences like this:  


  I doubt (this 100 yuan bill is fake)    


thus you are questioning the bill's..."fake-ness" which implies you think it's real  


  I suspect (this 100 yuan bill) is fake


thus you are distrusting this bill, thinking it's fake.

Now, to the translation:
我怀疑这张百元钞票是假的
is definitly the second explanation - I suspect this 100 yuan bill is fake.  

The way to express I doubt this 100 yuan bill is fake would be:
我对这张百元钞票是假的有怀疑

Hope this gives you an idea.