Someone offering an advisory service to traders has two sources of credibility:
i) they have a good profit record of trading for themselves - the customer can then expect to make money by replicating the advisor's methods
ii) they can't personally make a good profit trading, but are good at improving the performance of actual traders (e.g. trading psychologists & other coaches) - the customer expects to make money by benefiting from the coaching
If they can't either trade profitably themselves, or make other traders improve their trading, then their services are not really worthwhile IMO.
In case i), it is easy to prove credibility - just post account statements, or clear & thorough actual performance records, and get it signed off by an accountant. In case ii), it is a bit more difficult to prove credibility, however one easy way is to post the performance of, say, 10 randomly selected subscribers. If the service is good, then most of them should at least be profitable, and more profitable after being subscribers for some length of time than they were to begin with.
If the advisor or his clients DO have good performance, then there is absolutely no reason whatseoever not to post it - it can only help the credibility of the advisory service. So if such performance is not posted, then there is a strong suspicion that the reason for this is it does not exist.