openssl verify signature

Yes, you can use OpenSSL to create and sign a message digest of the plain text file and later use that signed digest to confirm the validity of the text.

openssl sha1 -sign rsaprivate.pem -out rsasign.bin file.txt

and later verify the validity of the text message using

openssl sha1 -verify rsapublic.pem -signature rsasign.bin file.txt

Check out the O'Reilly book Network Security with OpenSSL for a good documentation source for these functions. The example above came from that book.