User`s guide

DOS Books
The DOS Manual from Apple
Beneath Apple DOS by Worth & Lechner
Apple II User's Guide by Poole, Martin, and Cook
ProDOS Books
ProDOS Technical Reference Manual from Apple (Addison-Wesley)
Beneath Apple ProDOS by Worth & Lechner
Exploring Apple GS/OS and ProDOS 8 by Little
ProDOS Inside and Out by Doms and Weishaar
______________________________
From: Dennis Jenkins, Rubywand, Dave Althoff
041- How can a BASIC program tell which DOS it's running
under?
A pretty good way is to check the three-byte JMP command starting at $03D0.
If it's 4C 00 BE, you're running ProDOS; if it's 4C BF 9D, you're running DOS
3.3 or a compatible variant (such as Prontodos).
Two (moderately rare) factors may change the DOS 3.3 JMP: 1- On old "slave
disks" created on sub-48k Apple II's, DOS loads in at a lower address; 2- DOS
3.3 is sometimes moved up into the Language Card. So, your best procedure is
probably to check for ProDOS and, if it isn't ProDOS, just presume that it's DOS
3.3.