From the FAQs

We typically allow all giveaways to run their course without judgement, and allow the giveaway creator a fair chance to prove themselves and to come through with the gift in the end. An exception is when the giveaway creator enters a description that clearly states the giveaway is fake, or if the description illustrates a lack of understanding of how the giveaway system works. Please refrain from posting any comments accusing the giveaway creator of posting a fake giveaway, and only create a support ticket if the giveaway is fake beyond a reasonable doubt.

Expressing doubt about the sincerity of the creator in making his/her giveaway ("calling fake") is grounds for suspension. It is a fine line between "being helpful" and calling a giveaway into question, but most experienced users manage to avoid crossing it.