BinHex 4.0 files can be easily recognized since they begin with the line:

(This file must be converted with BinHex 4.0)

and are followed by a line starting with a colon, ':'.
The BinHex encoding of the file follows (64 printable characters per line), and is ended with another colon.

The following is from RFC 1741 Appendix A.
Here is a description of the Hqx7 (7 bit format as implemented in BinHex 4.0) formats for Macintosh Application and File transfers.

   The main features of the format are:   1) Error checking even using ASCII download   2) Compression of repetitive characters   3) 7 bit encoding for ASCII download   The format is processed at three different levels:      1) encoding of dir info, data fork, and resource fork into a stream of 8 bit bytes:
   Byte:    Length of FileName (1->63)   Bytes:   FileName ("Length" bytes)   Byte:    Version   Long:    Type   Long:    Creator   Word:    Flags (And $F800)   Long:    Length of Data Fork   Long:    Length of Resource Fork   Word:    CRC   Bytes:   Data Fork ("Data Length" bytes)   Word:    CRC   Bytes:   Resource Fork ("Rsrc Length" bytes)   Word:    CRC      2) Compression of repetitive 8-bit characters.   ($90 is the marker, encoding is made for 3->255 characters)   00 11 22 33 44 55 66 77   -> 00 11 22 33 44 55 66 77   11 22 22 22 22 22 22 33   -> 11 22 90 06 33   11 22 90 33 44            -> 11 22 90 00 33 44      3) Conversion into a different stream of "safe" 8-bit bytes as follows:
The whole file is considered as a stream of bits. This stream will be divided in blocks of 6 bits and then converted to one of 64 characters contained in a table. The characters in this table have been chosen for maximum noise protection.
The format will start with a ":" (first character on a line) and end with a ":". There will be a maximum of 64 characters on a line. It must be preceded, by this comment, starting in column 1

(This file must be converted with BinHex 4.0)

Any text before this comment is to be ignored.
The characters used is: !"#$%&'()*+,- 012345689@ABCDEFGHIJKLMNPQRSTUVXYZ[`abcdefhijklmpqr

(These are not the same as those used in Base64 (those regarded as safe according to the MIME standard, RFC-1521)).