« file1 -nt file2
    True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2
    True if file1 is older than file2, or if file2 exists and file1 does not. »

Application
if [ "/path/to/file1" -ot "/path/to/file2" ]; then
    echo "FAIL"
fi