Takes an open file and tries to determine the endianness of a Seismic Unix data file by doing some sanity checks with the unpacked header values.

Returns False if the sanity checks failed and the endianness otherwise.

It is assumed that the data is written as 32bit IEEE floating points in either little or big endian.

The test currently can only identify SU files in which all traces have the same length. It basically just makes a sanity check for various fields in the Trace header.