Hullin, Chapter Ten, Mishnah Four
1) A convert who converted and owned a cow:
a) If he slaughtered it before he converted, he is exempt from giving the gifts.
b) If [he slaughtered it] after he converted, he is liable.
c) If there was a doubt about it, he is exempt, for the burden of proof lies upon the claimant.
2) What is the shoulder? From the joint up to the shoulder-socket of the forelimb, and this is the same for the nazirite.
3) The corresponding part of the hind leg is called the thigh.
a) Rabbi Judah says: the thigh extends from the joint up to the fleshy part of the leg.
4) What counts as the cheek? From the joint of the jaw to the last protrusion of the windpipe.
Section one: If the convert was not yet Jewish before his cow was slaughtered, he is exempt from the gifts. If, however, he converted and then slaughtered the cow, he must give the gifts. We can see that the critical time is when the animal was slaughtered.
If there is a doubt whether the conversion occurred before the animal was slaughtered, then the priest must prove that the owner was Jewish before the slaughtering in order to be able to claim his gifts. This is based on the common legal principle that the burden of the proof is on the claimant.
Section two: The end of chapter ten is concerned with identifying exactly what part of the animal constitutes the shoulder and cheeks. The mishnah does not discuss the identification of the stomach, perhaps because it is clear what constitutes the stomach.
The shoulder is defined as the entire front right leg, except for the foot. The joint refers to the ankle. This is the same shoulder as referred to in connection with the Nazirite in Numbers 6:19, And the priest shall take the cooked shoulder.
Section three: The same part of the hind leg is given to the priest from the shelamim offering. See above, mishnah one.
Rabbi Judah says that the thigh referred to in the Torah is really what we would call the lower leg. It does not include from the knee and upwards.
Section four: The cheeks go from the lower cheeks until the first joint of the windpipe.