For donations to qualify for income tax purposes, the charities have to be registered with their countries' tax authorities. Donation receipts will have the charity's registration number printed on them, have a receipt number and year or date of donation. Some charities issue tax receipts upon receiving donations or wait until their year-end time to send out receipts for all donations sent in the year-the availability of staff to do the paperwork can determine the timing of the receipts.
Be aware to check out the validity of charities and if you donate after a telephone call, ask during the call who is issuing the tax receipts. Some charities may outsource the telephone solicitation to another company, which generates the receipts on behalf of the charities. However, make certain to keep a record of the date of your donation, the amount and the charity. Follow up before the year-end. Why? I once worked at a non-profit that had outsourced the solicitations and had to take calls from donors who did not get their receipts by tax time-they were upset and the absence of receipts also affected their perception of the non-profit. Later, the responsible company just dropped off hundreds of unmailed receipts for the non-profit to send out. Ask first and keep a hold on your funds if you are in doubt about the method of solicitation, even if it is occurring at your front door.
In Canada, charities register with the Canada Revenue Agency (CRA): http://www.cra-arc.gc.ca/tx/chrts/menu-eng.html
In the United States, charities register with the Internal Revenue Service (IRS): http://www.irs.gov/charities/article/0,,id=96136,00.html
For Australia, rules on gifts and donations to non-profits are given by the Australian Taxation Office.
United Kingdom has a Charity Commission.