This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Apple II Upon Reset, the original Apple II starts you in the system monitor looking at the "*" prompt. It allows step execution of machine code and has Integer BASIC in ROM. The major division between kinds of Apple II is Revision 0 and Revision 1. The Revision 1 motherboard adds a number of features including a few which are easily observed: Power-On Reset: The computer automatically does a Reset when turned On. More hires colors: To the Black, White, Violet, and Green available on a Rev0 machine, Rev1 adds Blue and Orange. Color Killer added: Full-text displays are black&white without the color fringing and tinting you see on Rev0 machines. Apple II+ All Apple II+ machines have the Revision 1 or higher motherboard and the Autostart ROM. On power-up the Apple II+ does a Reset and displays "APPLE ][" at the top of the screen. If a disk drive is connected, the II+ will try to boot a diskette. The Apple II+ loses some monitor features (like instruction stepping) and in-ROM Integer BASIC found in the earlier Apple II; but, it gains the more powerful Applesoft BASIC in ROM. A II+ Reset normally leaves you in BASIC looking at the "]" Applesoft BASIC prom pt. Apple IIe You can usually tell a IIe from a II or II+ by the nameplate. On models with the classic Apple II case but no nameplate, you can check the keyboard. IIe models include a key embossed with the outline of an apple called the "OpenApple" key located near the bottom left corner of the keyboard. (All later Apple II's have this key, too; but, they do not look anything like a II, II+, or IIe). A few IIe models produced for third parties may have some other special-logo key in place of OpenApple. Within the IIe series, the major division is between Enhanced and unenhanced IIe models. Look at your computer while booting. If it says "Apple ][", it is not enhanced. The enhanced computers will say "Apple //e". Today, "Enhanced IIe", "//e", and "128k Apple IIe" are used interchangably because nearly every Enhanced IIe has an Extended 80-Column Card plugged into the 60-pin Aux Slot (which adds 64kB of RAM). Technically, an Enhanced IIe is defined by the presence of three or four IC's: the 65C02 microprocessor (replaces the 6502), new Character (or "Video") ROM which includes MouseText characters, and new monitor firmware in ROM. If a IIe has the 65C02 microprocessor, it is probably an Enhanced IIe. If your IIe is not enhanced, you can do the enhancement yourself with an "enhancement kit" consisting of the four chips you need to swap in. The last significant upgrade to the IIe series came in 1987 with the release of the Extended Keyboard //e. This model is a 128k Enhanced IIe-- it comes with an Extended 80-Column Card plugged into the Aux Slot-- which adds an 18-key 'numeric keypad'. It also replaces the eight on-motherboard RAM chips with two 64kx4 IC's; and, it replaces the two BASIC/monitor ROMs with a single large ROM. Quite a lot of later 80's 8-bit software, including all double-hires software, requires a 128k Enhanced IIe. (If you have a //c, IIc+, IIgs, Laser 128, or Franklin Ace 2000-2200, you have good to at least decent Enhanced IIe compatibility.) Unfortunately, a small number of early-release IIe's can not be upgraded to handle double-hires. Check the serial number on the motherboard (in the back, by the power-on led). If it is 820-0064-A, you must change the motherboard to upgrade (unless you have the PAL video output version). The IIe was produced in very large numbers and sold around the world in countries with different power systems using different video standards. So, it is not all that unlikely that you may need to check a bargain IIe to make sure it will work in your home using your monitor. The two major video output formats you may run into are NTSC (used in the US, Canada, Japan, and most countries with 60Hz power, except Brazil) and PAL (used in Australia, most of Europe, and most countries with 50Hz power). One w ay to tell which video standard a IIe uses is the location of the Aux Slot. If it is on the side of the motherboard near the power supply, you have an NTSC model. If it is in line with Slot 3, you have a PAL model. Apple //c and Apple IIc+ Go into Basic and type "PRINT PEEK (64447)" and press return. If it says 255, you have a very old //c. This model is known to have problems producing accurate baud rates for serial communications. It's been many years since the / /c was released; but, some long-time Apple dealers may still perform the upgrade for a nominal fee. (Tell the dealer that the Apple authorization number is ODL660.) If PRINT PEEK (64447) displays 0, you can use 3.5" drives, but you don't have the memory expansion connector. If it says 3, you have the memory expansion connector and can plug in extra memory. If it says 4, you have the latest model of the //c with the memory expansion connector and other upgrades. If PRINT PEEK (64447) displays 5, you have an Apple IIc+. The IIc+ also has "IIc Plus" silkscreened in dark gray onto the upper right corner of the case. Apple IIgs There are 3 major versions of the GS: Check the initial power-up screen. It will probably say ROM-01 or ROM 3. If it does not say either, you have a ROM-00 model. You must upgrade a ROM-00 machine in order to run current system software. The ROM-01 has 256K on the motherboard, while the ROM 3 has 1 MB on the motherboard. Most of the enhancements of the ROM 3 are added to the ROM-01 simply by booting up with current system software. --Dan DeMaggio, CreatSltn, Steve Leahy, Nathan Mates, Bevis King, Davi d Empson, Jeff Blakeney, David Wilson, Rubywand