The lot number identifies when a product was manufactured. It will be in small type, all capital letters, and say "LOT #". On many products, it's on the back panel. On kit boxes, it's either on the top or bottom of the box, generally affixed to a label.
Lot numbers have 2 parts: a letter, followed by a series of numbers. The letter corresponds to the month the product was manufactured: (A = January, B = February, C = March, D = April, E = May, F = June, G = July, H = August, I = September, J = October, K = November, L = December).
For the numbers, the first number third digit of the year the product was made. The next 2 numbers represent the day of the month the product was made. The next number represents the last digit of the year the product was made. The next two digits represent month the earliest expiring component expires. And the last two numbers represents the year the earliest expiring component expires. Example: LOT # B-01580609 means that the product was manufactured on February 15, 2008 and the earliest expiration date for any item in the kit is June, 2009.