How do you test methods when their output is determined by data private to their class?

I have this service which is responsible for creating and verifying JWS tokens in my project.

@Service
public class TokenService {

    private static final KeyPair KEY_PAIR = Keys.keyPairFor(SignatureAlgorithm.RS256);

    public String create(String subject) {
        return Jwts.builder().setSubject(subject).signWith(KEY_PAIR.getPrivate()).compact();
    }

    public Jws<Claims> verify(String token) {
        return Jwts.parserBuilder().setSigningKey(KEY_PAIR.getPublic()).build().parseClaimsJws(token);
    }

}

How can I verify that the create method returns the correct token for a given subject when I don't know what the private key it uses is?