How to find the revision number of Atari Basic#

With the BASIC cartridge put into your 400/800 or BASIC switched on, type (e.g. after the READY prompt)

? PEEK(43234) 

and then press RETURN.

The computer will give you an result.

162Revision AFirst Atari BASIC cartridge, 8K ROM.
96Revision BFound built-in on the 600XL and early 800XLs. Never supplied on cartridges.
234Revision CFound on later 800XLs, the 800XLF, XEGS and all XE computers. Limited cartridge production run.

For an description of the known errors, see the Wikipedia Article on ATARI Basic:

See also OS Versions