Magic as a way to determine the truth should never be final evidence just a truth-tests in RL will not lead to a conviction if it is the only evidence. As a matter of fact, even cameras and photos can be etchy as evidence since they are relatively easy to tamper with. It is just as easy to fool magic with equal or lesser magic.
On a side note, I doubt magic would have a real big influence on society as long as those who wield magic are relatively uncommon. Low level spellcasters can prepare only so many spells. Besides, they can also be at so many places at the same time. Sure, a mage with a fireball would be like a soldier with a grenade launcher with ONE grenade. He will have an impact on the battle, but the impact (unless used very strategical) will not spell victory or doom. This is especially true with clerics who tend to compete with clergy of other faiths and a higher power that controls there magic. Somehow I doubt most religions would deal with death by selling life-insurances. The after-life exists, who are mere mortals to keep an other mortal from his just rewards?
On a side note, I doubt magic would have a real big influence on society as long as those who wield magic are relatively uncommon. Low level spellcasters can prepare only so many spells. Besides, they can also be at so many places at the same time. Sure, a mage with a fireball would be like a soldier with a grenade launcher with ONE grenade. He will have an impact on the battle, but the impact (unless used very strategical) will not spell victory or doom. This is especially true with clerics who tend to compete with clergy of other faiths and a higher power that controls there magic. Somehow I doubt most religions would deal with death by selling life-insurances. The after-life exists, who are mere mortals to keep an other mortal from his just rewards?