"If" is not always the sign of a question ( i ) ^_____^

It's a statement, so I'd use a period. Though, I've seen some people who do add the question mark to indicate the doubt. I don't advise doing that, it tends to confuse people. :)

Usually it would be a period, as Kei says. It can also be a question mark if it is expressed as a question, such as in reported or verbal speech. " (are you suggesting that) I don't really know if I can do it or not? " If it's written, context will be the key. If it's spoken, then it depends on which words are emphasised in the sentence.

i think period (.)