No one who lacks knowledge of a subject is competent to pass judgment on that subject.
Conclusion
Only seasoned politicians are competent to judge whether a particular political policy is fair to all.
Evidence (since)
No one who lacks knowledge of a subject is competent to pass judgment on that subject.
Political know-how is a matter insight and style learned through apprenticeship and experience.
Evaluate
If we're coming at this from the anti-conclusion position, we're arguing,
The author will cite his rule for who is "competent to judge": in order to be competent to judge, you must have knowledge of the subject!
Okay, we're talking about whether someone would be competent to judge the fairness of a political policy, so, in this case knowledge of the subject would probably mean something like knowledge of the policy details and knowledge of what is fair?
Do we think that only seasoned politicians have knowledge of the policy details? Of course not, any politician or anyone who gets a copy of the proposed legislation can have knowledge of the policy details.
Do we think that only seasoned politicians have knowledge of what is fair? Of course not. That's something we all have knowledge of.
The other premise is trying to establish that only seasoned politicians will have political know how, because political know how requires experience. "Seasoned" is a synonym for "experienced".
So the evidence has established that only seasoned politicians have political know how, but it hasn't established that only seasoned politicians have knowledge of what is in this particular policy or whether that policy would be fair.
Goal
Let's look for an answer addressing that disconnect, or look for an answer that helps us argue that other people would also be competent to judge whether a policy is fair.