The 4th byte in the SWF file carries the version number, for example 0A is for Flash Player 10.
EDIT: Because of the high interest this question got I’ve decided to give more feedback
The first 8 bytes of any SWF file are not compressed, the rest of the file could be compressed (or not) by zlib compression.
- 1st byte: ‘F’ (not compressed) OR ‘C’ (compressed).
- 2nd byte: ‘W’ always.
- 3rd byte: ‘S’ always.
- 4th byte: version number (09 means this file is targeted at Flash Player 9 and so on…)
- 5th to 8th: Length of entire file in bytes.