When you give to a charity, you are, in effect, buying a small slice of a worthy cause.

It's a purchase like anything else, but many donors don't know what they're buying when they donate to a charity. Sure, if you give to your college, local hospital or religious organization, you're pretty sure of how your money is being used. But what about all those emotional appeals that come in the mail? Or the people who call on the phone or ring your doorbell?

And what about that more recent species of money-seeking: the non-charity appeal for a supposedly good cause. Special interest or political lobbying groups are having a field day: You can find all sorts of pitches for such causes as antihomosexuality, lower taxes, women's rights, anti-abortion (or freedom of choice in the matter) and various anti-union or anti-business programs. Anita Bryant has organized an appeal to "Protect America's Children" (which is not to be confused with Save the Children Federation).

The success of Proposition 13 (the anti-property tax law in California) has created new markets for appeals from The National Tax Limitation Committee, which says it's against high taxes and wants reductions at state, local and federal levels. A handbook on how to fight high taxes is sent ot each donor.

What isn't sent - or available - according to the Philanthropic Advisory Service of Council of Better Business Bureaus, is adequate financial information with which to judge The National Tax Limitation Committee.

The Better Business Bureau people also say that Anita Bryant's group has not provided enough financial information either.

Whether you're giving to an organization that helps starving children or one that is formed to lobby for a political cause or a special interest, you should get enough facts on which to base a yes or no decision.

Always ask for a financial report accompanied by an independent audit. If this is not forthcoming (or if there are reservations in the independent audit report), you might as well forget about making a donation.

As a general rule, a well-run philanthropic organization or political action committe should pay out a lot more money to the cause than it spends on fund-raising. That means, the experts say, the organization should spend around 75 percent of its income on the needy cause it espouses. If it's a 50 percent or less, then you have to wonder if they're not just building their own high-salaried little empire.

But telephone solicitation - another bad practice - is on the upswing. Unless you know the person who's calling, it's best to hang up. Or, if you're curious, ask the person on the other end of the line to send you a statement of purpose and financial report (with independent audit). Many of these telephone pitches are fronts for straight profit-making organizations.

You can get an honest rating of hundreds of charities from two organizations with impeccable credentials: Philanthropic Advisory Service, Council of Better Business Bureaus, 1150 17th St. NW, Washington, D.C. 20036 (include $1 and a self-addressed, stamped long envelope); and, National Information Bureau, 419 Park Avenue South, New York, N.Y. 10016 (send a postcard with your address). They'll also give reports on individual charities.