I don't have a hang of a lot more to add - just that if you have concerns that she is the kind of person who would use a serious medical condition to emotionally blackmail you, then you should run away as fast as you can - whether she actually has ms or not is irrelevent to how you feel about her. A good, healthy and productive relationship is based on honesty and trust, as well as love - and at least the honesty and trust are missing here.
I don't think it is really important that you pin her down on the ms issue - you don't need to prove her a liar in order to walk away, nor do you want to be compelled to stay with her out of guilt if she does have it. But if you have to 'casually' call her on it - through out some of the common medical terms - lesions or plaques, areas of the brain such as peri-ventricular, brain stem etc, sub-cortical, white matter as opposed to grey matter, ask about her MRI (as previously suggested) - how many lesions she had (then ask again a few days later - she might not remember what she said before!), T1 & T2 lesions, interferon treatment, iv steroids etc.
Everyone else has given you great advice - listen to your instinct...