Assuming that x is an integer, the construct if x: is functionally the same as if x != 0: in Python. Some languages' style guides explicitly forbid against the former -- for example, ActionScript/Flex's style guide states that you should never implicitly cast an int to bool for this sort of thing.
Does Python have a preference? A link to a PEP or other authoritative source would be best.
Typically, I read: 
if(x) to be a question about existence. 
if( x != 0) to be a question about a number.
Which of `if x:` or `if x != 0:` is preferred in Python?