You should be thinking of output encoding, rather than output validation.
Output encoding is about properly escaping special characters so they appear correctly to the user, rather than getting interpreted with their special meaning.
For example if your field contains
One is < Two, and you are showing this on a HTML page, then you need to encode it as
One is < Two. To do this with ESAPI, you would use the encodeForHTML method.
The characters that you need to encode, and the way to encode them will vary depending where you are using the data.
For example, if you are using
One is < Two as part of a URL, the spaces also need to be encoded, and the less than sign becomes
%3C, and the result will be
One%20is%20%3C%20Two. To do this with ESAPI, you would use the encodeForURL method.