Lift the hinged panel of the Motion Detector 2 and look at the underside. You will find a series of numbers, a gap, then a letter followed by a dash and four numbers, and a final letter.

For example:
2312000363 P-0712D

The four digits after the dash represent the month and year of manufacture, so the example number is from a unit manufactured in month number 07 in the year 2012, or July 2012.

See also:
Which Motion Detector model do I have?