I don't factor in sexual attraction in my perception of credibility. Attention, sure, but credibility? No.
I use different nonverbal cues to ascertain credibility.
Speech patterns are very telling, usually. When someone repeats a word often, such as "actually" or "unfortunately", they are demonstrating either nervousness or a small vocabulary. Neither is a sign of credibility, imho. Poor language either communicates comfortability or an emotional response of some kind (and also possibly a limited vocabulary).
Eye movement is absolutely key. Where a person looks while communicating says more than the words they are saying, usually.
Top left = visually imagined images
Top right = visually remembered images
Left = auditory imagined
Right = auditory remembered
Bottom left = smell, feel, taste
Bottom right = internal dialogue
Whether this is innate, I don't know. I learned about this back in high school psych class and again in college (you can find info online about it, I found the image here:
http://www.blifaloo.com/info/lies_eyes.php)
Dress? Meh.