You can use the file
utility to give you an indication of the type of line endings.
Unix:
$ file testfile1.txt
testfile.txt: ASCII text
"DOS":
$ file testfile2.txt
testfile2.txt: ASCII text, with CRLF line terminators
To convert from "DOS" to Unix:
$ dos2unix testfile2.txt
To convert from Unix to "DOS":
$ unix2dos testfile1.txt
Converting an already converted file has no effect so it's safe to run blindly (i.e. without testing the format first) although the usual disclaimers apply, as always.