There is an easy way to determine the date of your vintage Jensen speaker. On the edge of the speaker where the mounting holes are there will be a six digit number. The first three numbers will be 220 which denotes the manufacturer as Jensen. The next number is the year of manufacture and the last two numbers will be the week of manufacture.
Take the following example:
In this case, "220" is the Jensen manufacture code. "07" represents the year (2007), and "46" indicates that it was made in the 46th week of the year. Unfortunately on some older Jensen speakers, the year is only represented by a single digit (just "7" for example). In that case, you will have to make a guess as to the decade in which the speaker was produced (1947, 1957, 1967, etc.). If your speaker is in its original amplifier, you can make an educated guess using information from your amp.