Start with the register
Every charity operating in England and Wales with an income above £5,000 must register with the Charity Commission, which runs a free public search at gov.uk. Enter the name and you'll see the registration number, the stated purposes, and whether the charity is still active. Scottish charities register with OSCR; Northern Irish ones with the Charity Commission for Northern Ireland. If a charity soliciting your money doesn't appear on any register, that's reason to stop.
Registered charities must file annual accounts and trustees' reports, and the Charity Commission publishes them
Read the accounts
Registered charities must file annual accounts and trustees' reports, and the Charity Commission publishes them. You don't need to be an accountant: check when accounts were last filed (badly overdue is a warning sign), look at the income and how it's spent, and read the trustees' report to understand what the charity actually did that year. A legitimate organisation will show its working.
Spot the warning signs
High-pressure asks — donate now or the moment is lost — are a red flag. So is vague language about where money goes, reluctance to provide a registration number, and names that closely mimic well-known charities. Unsolicited contact out of nowhere warrants extra scrutiny. The Fundraising Regulator handles complaints about fundraising behaviour in England, Wales and Northern Ireland; the Charity Commission can investigate serious concerns. Neither guarantees every charity is effective, but choosing a charity you can trust involves both confirming it's genuine and checking whether it spends well — two different questions worth asking separately.