Perhaps that was true at one point. Initiation into Dragon Age: Inquisition negated that belief from the beginning scene in Hinterlands. Practical illustration includes: Cassandra calls to the Templars attacking the crossroads, "We are not apostates!" Solas observes, "I do not think they care." Another example: I took a promise ring off of a dead Templar and gave it back to the wife of the victim - she said the Templars couldn't tell the difference between a shovel and a mage's staff and had to take the wedding ring off of victim's dead finger in case it was magic. Once Templars are cleared from the road, people start remarking that refugees and innocent travelers will be safer.
But lets entertain the idea that the necessary purpose of the Templars is to watch the Mages and prevent them from corruption. Who then watches the Templars and the Seekers and prevents them from corruption? The question presented, I believe, is an old one.
Quis custodiet ipsos custodes?
See also: in the vast majority of cases.
As for your question of who guards the guards, you've managed to hit the right question, and managed to hit exactly the problem I have with Xilizhra's suggestion.
Keeping the Templars in line was the Seekers' job, which they failed at due to their sympathizing more with the Templars than with the Mages or the Priests. Essentially the original problem was that, at least in practice, the Templars answered to themselves rather than any outside group; that was a problem then and will be again if the people who are supposed to keep mages from threatening others answer either to themselves or to the mages. Then the Seekers decided they no longer answered to the people who were supposed to keep them honest, and the Chantry couldn't do anything about it because their ability to enforce their will came from the Templars, which displays another aspect of the problem: namely, the fact that if you aren't a military power, you aren't going to be able to guard the guards if they say you can't.
With all that in mind, I think the only solution is to hope to the Maker the Inquisition sticks around for a while: it's the only group that has a prayer of doing this job properly.
Actually, that would work fine for me. I wrote a lengthy reply advocating exactly that, once, as a hypothetical solution. I'm not even remotely convinced that magic is a positive force in Thedas, or that most of the time the world wouldn't be better off without it.
I'm all for completely removing mages to their own area and letting them Isle of Dr. Moreau themselves into abominations all day, just as long as nothing tries to get in or out of the enclave.
Are the Wardens exceptions to these rules, or do they need to do without mages in this hypothetical? Because while we don't know that the mages are necessary for the Joining, we know they're used in it, and if it was for any reason other than there being a large benefit (and possibly an necessary one) to the person doing the prep-work having magic you'd think the Wardens would learn to do it themselves. Since, you know, what they're doing instead is letting the Circle in on a secret ritual that might be worth a tremendous side-eye from most of Thedas and a good portion of the Templars. (And that's not counting the benefits of having mages to actually use against the darkspawn.)