While following up on an entry in the current Time cover story on the 25 Most Influential Evangelicals, I found this interesting reference to a group called Wall Watchers. I have no idea whether they are effective but they have an admirable goal of promoting transparency in Christian charities, verifying that money given actually goes to the causes claimed. From their Purpose page:

Wall Watchers' MinistryWatch service will increase the confidence of donors by providing an independent source of information on Christian ministries to aid them in making their giving decisions. Greater knowledge of the good works being done by Christian ministries and insightful analysis of the activities of these ministries can only lead to higher levels of giving by those who feel called to financially support God's work.

MinistryWatch currently has overs 500 Christian charities and also currently has info on some of the tsunami scams. This doesn't entirely dispell the suspicion I feel about many televangelists's legitimacy and how much money actually gets to charitable works but it's a great resource for checking out these charities.

Not being Christian myself, I would gravitate more to the secular charities. For another similar watchdog group, try Charity Watch which also checks on the effectiveness of charities.