Difficult, indeed. And yet, there are Caracalla Caesar denarii that could have been the models for yours (attached). Also, usually you can perceive that Diadumenian will have his father's nose, while Caracalla's brow slopes more. Pat L.
It is this one:
• 15 03 01 AR denarius. Caracalla as Caesar, draped bust to r. M AVR ANTONINVS (divided over head?) CAES. Rev. Minerva stg l. leaning on reversed spear and resting r. on her shield. SECVRITAS PERPETVA
. RIC 2 and, with a different obverse die, pl. XI, 7.