My church's lesson manual for this week's study explains the difference between fiat religion and true religion pretty clearly:

"[Jesus] taught ... that the true purpose of religion is not to create burdens but to relieve them. He taught that God gives us commandments, including the one to honor the Sabbath, not to oppress us but to bless us. Yes, the way to God is strait and narrow, but the Lord came to announce that we need not walk it alone. 'Come unto me,' He pleaded. His invitation, to all who feel 'heavy laden' for any reason, is to stand beside Him, to bind ourselves to Him, and to let Him share our burdens. His promise is 'Ye shall find rest unto your souls.' Compared to the alternatives—trying to carry on alone or relying on mortal solutions—His 'yoke is easy, and [His] burden is light.' "

Any religion that says you should trust in a man, or that adds greater burdens instead of relieving you of the burdens you already carry, is fiat. But to know whether a religion is fiat or true, you need to look at it more deeply than a quick superficial glance. Like with the scientific method, the only way to verify it is to test it out yourself.