It can be tough. But if it really was a work injury should be fine. it is easiest if it was a very clear injury, like something falling on you, and you had 10 witnesses. Dealing with overuse injuries gets a bit harder, since from their perspective there isn't really any way to prove the injury is from work.
It always seemed to me that they were almost psychotically paranoid about fraud and my advice to that would just to be very consistent with your account of the injury and to keep very detailed records of things (both in terms of the injury and in conversations you have regarding the claim - i.e. when you talked to who and what was said). And that advice includes talking to coworkers and friends - these days you never know who's going to hear what you think are private conversations. This isn't to imply you have anything to lie about, it is just that I think even small discrepancies can arouse suspicion (because they are so sensitive to them).
In my case, what I think made it go pretty smooth was the fact that my boss was completely supportive of my claim. In the absence of that I think it would have been much tougher. If your employer is hostile to your claim and it is a significant injury, I would probably advice getting a lawyer. Fighting through an employer by yourself can be a very difficult challenge.