Well, assuming I don't have out of band information like the file length.

For instance:

: pts/5[r] bash[7183] ; md5sum /tmp/file
0cc175b9c0f1b6a831c399e269772661  /tmp/file
: pts/5[r] bash[7184] ; ls -l /tmp/file
-rw-rw-r--    1 jdamery  jdamery         1 Aug 28 11:26 /tmp/file

And you ought to be able to discover what the contents of /tmp/file are without *too* much effort.