We reduce can when it is used with another verb. It sounds like "ken" or "kn." For example, we say "I ken do that, I kn see it." We don't reduce can't, so it will have a clear vowel /æ/.
When there is not another verb (e.g., "yes, I can" or "no, I can't."), can is usually longer in duration, while can't is more abrupt. "Yes, I caaaan." "No, I can't."