Its a good a question. When a new signals service is created earning some credibility will take time. When ddmarkets forex signals launched in 2014 it was actually free of charge for half a year. After 6 months it turned into a paid service, whoever wished to keep receiving the signals signed up.
It's same concept here. There are dozes of crypto signals out there. Websites, telegram, twitter, facebook etc. When the signal is given for free for a limited period of time (it will not be 6 months) it earns some credibility.
Many crypto traders that trade in an exchange, not a trading platform lie MT4 or MT5 don't use any signals. They buy and sell as they wish. The aim of the signal is to increase the accuracy of the trades. Traders that leverage crypto will probably use signals due to the high risk. Finding a legitimate signal provider is pretty tough.
So to answer your question, it's credibility.