How to determine if your butt implants were placed over the muscle, subfascial, intramuscular, or submusclar?

Indeed ultrasound could be used as a possible non-invasive modality of determining the tissue level/depth of buttock implants.  However, ultrasound is only as good as the technician operating the device and doctors ability to read the ultrasound images. Thus an MRI scan would in general be more sensitive and specific.  That all being said, a physical exam by an expert in buttock implants/revisions is usually at least 90% accurate. 

Dear Practical6007,
there is a number of different methods to determine the exact implant position. Ultrasound and MRI are some of them. If you have any concerns, I suggest you to discuss this with your plastic surgeon. 
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

The most definitive and unequivocal method to determine the location of your buttock implants is an MRI. Physcial examination and ultrasound will provide some information but the clear images of the implant and the muscle seen on an MRI provide the most accurate information.