Try this:

if [ `find xfile -newer yfile` ]
then
echo "xxx"
fi

In the above we are testing for a string to be returned.

If xfile is newer than yfile then the find statement will return the string "xfile", hence the test will be true and perform the if statement.

If xfile is not newer than yfile then the find statement will not return anything, so the test will be false and the if statement will not be performed.

Let me know how you get on.

Cheers
Helen