Between the data that is available and my own personal knowledge gained from having gone through over a thousand preban AR15 magazines there are three common ways to identify 30 round preban AR-15 magazines.
The first of these methods and most reliable would be an external date stamp. The external date stamp will appear in a format of “xx/xxxx” representing the month and year of manufacture. This is the ideal magazine you would want for identification purposes. An externally date stamped magazine will look like the following:
The second method to identifying preban magazines, and the second most reliable method, would be an internal date stamp. Internally date stamped magazines are about as easy to find as externally date stamped magazines but the floor plate must be removed to verify a preban internal date stamp. With an internal date stamp it is common for both side of the magazine to be stamped with a code. These codes usually look like the following, “K4 87” on one side of the magazine and then “04 87” on the opposite. The “K4” represents the month of manufacture, the number follow the “K” will be the month, so in this example the “4” represents a magazine manufactured in April. The “87” stamp represents the year of manufacture, so this magazine was manufactured in April 1987. These stamps can appear horizontally, vertically, or even upside down when stamped inside the magazine so if you were to see a date stamp code of “K11 98” this is mostly likely a November 1986 date of manufacture due to the fact that the 86 can be stamped in any direction. Some externally date stamped magazines will also feature an internal date stamp, this is how I have verified that the internal date stamps do represent the dates of manufacture as well as confirming that the internal date stamps are stamped in multiple directions (vertically, horizontally, upside down, etc.).
The third way to identify, and the least reliable is by the lack of a date stamp. Since 1994 manufacturers have been stamping their post ban magazines with stamps that say "LEO/MIL USE ONLY", etc. along with an external date stamp. Do note though that there is a magazine manufacturer who currently produces post ban magazines that lack the MIL/LEO warning and also lacks any date stamp. Do not purchase these magazines because they are illegal regardless of whether they look identical to preban magazines or not. There are people here in NY on other forums that will tell you these magazines are preban, if they look brand new and have a damaged baseplate chances are the magazine is a postban magazine that has had a preban floorplate put on it by someone to make the buyer think they are getting preban magazines in excellent condition with just a worn base plate.
However, the majority of preban magazines lack a date stamp so 90% of the preban magazines in existence will not have any type of marking and these are legal preban magazines so don't hesitate to buy these if you get the opportunity.
If you have any questions about any of this stuff just ask.