Hi and welcome
Q; How do I determine whether it’s MS or not?
A: A neurological condition like MS is highly unlikely to be the cause of your 10-15 second thigh numbness, when you have zero brain and or spinal cord lesions showing up on your MRI....your diagnostic evidence wouldn't actually be suggestive or consistent with a diagnosis of MS so it's much more likely to be caused something else.
MS doesn't typically cause very short periods of symptoms like your saying you have been experiencing, and because 10 - 15 seconds is a very short time frame to be able to register that you don't feel anything (numbness) in your thigh its possible the sensation youve been experiencing isnt numbness or nerve related but something more soft tissue or muscle related intead.
Is this abnormal sensation always in the exact same location?
Hope that helps.....JJ