Hi Samuel,
I use a really simple method,
The thyroid gland sits relatively low in the neck and only the larger nodules or goiter type glands will have retrosternal extension.
As you scan down the right or left lobes of the gland in an axial plane, eventually your transducer will reach the medial head of the clavicle and you will generate an acoustic shadow from the bony structure.
If you have not cleared the lower pole of the respective lobe then you have retrosternal extension. If you are scanning down the midline, following the isthmus part of the gland inferiorly you will arrive at the supra sternal notch and then the manubrium of the sternum will be your bony landmark. Apply the same principle here.
The exact amount of retrosternal extension may be difficult or impossible to estimate with ultrasound and cross sectional imaging such as a CT scan can easily provide this information if required.
Happy scanning,
Steve