The answer should become a bit clearer whenever the government gets around to tightening up policing of the sector. The long-overdue Charities Regulation Bill is due to see the light of day sometime next year and, assuming it is passed into law before the general election, will lead to the establishment of a regulator with powers to force charities to declare how much they collect and how of the money is spent.

Until then, it's pot luck. Organisations such as Focus Ireland and Concern are upfront about their finances, winning plaudits from the Leinster Society of Chartered Accountants for the quality of their financial reporting. But only charities that are registered as companies are required to publish accounts.

The others can keep their finances secret, even from donors and supporters.

This makes it all but impossible to identify spendthrift charities from those who run a tight ship. Concern reported that management, administrative and governance costs accounted for 1% of total expenditure in 2005. Was this a little or a lot? It's hard to judge when other charities refuse to be similarly forthcoming.

Even when the promised regulator forces all charities to open up their books, picking out the ones that give most bang for your buck will be no easy task says Freda Donoghue, senior research fellow at the Centre for Nonprofit Management at Trinity College.

"The standard ways of measuring value for money don't necessarily work with charities because they've all got different ways of measuring inputs and outputs, " she says. "Some use volunteers while others have paid employees. For example, how do you measure the contribution of members of religious orders to the many charities in which they are involved?"

These were the type of obstacles encountered by the Comptroller and Auditor General when he tried to investigate the value the government got for the 877m it gave to nonprofit organisations in the disability sector in 2004. One of the organisations investigated had not filed accounts for years, despite receiving 288m of state funding between 2000 and 2004.

The state spending watchdog found special problems in identifying where all of the money was going. It concluded: "Expenditure in the nature of headquarters' costs and overheads was not generally reported in the financial statements in a transparent manner. . . . Similarly, remuneration packages of executives and management of nonprofit organisations were not generally disclosed. Greater transparency in financial reporting would provide assurance that charges were reasonable and would assist in allaying any concerns that funds might be absorbed by administrative costs rather than applied to front-line services."

Even if charities tightened up on bookkeeping, Donoghue is not convinced that we would change the way we give to charity.

"The perception is that, if charities have a lot of overheads, people will give less, " she says. "But in general a lot of charitable giving in Ireland is ad hoc and unplanned, prompted by door-to-door collections or street collectors. So perceptions about high overheads don't always influence us in practice."

While picking the best charity might be the hard bit, deciding how to give your cash should be a lot easier. This is because both you and your chosen charity would be a lot better off if you made your donation tax efficiently.

By making use of Revenue-approved schemes, you can make sure your donation is boosted by money that would otherwise disappear in tax.

People in the PAYE net do not get to pocket the tax relief themselves; instead, the breaks go to the charity, which can claim back the tax that the PAYE donor would have paid on the money. If you pay tax at the top rate, this means that a 250 donation is worth 431 to your chosen charity after it claims back the tax you paid on the money. If you pay tax at the lower 20% rate, your 250 donation is worth 312 to the charity.

Self-assessed taxpayers are treated more favourably because they get to pocket the tax relief themselves, rather than it going to the charity. This means that a 250 donation will only cost them 145 after tax, assuming they are in the top 42% tax bracket. Corporate donors are in a similar position because they can also write off charitable donations as a taxdeductible expense.

Like all tax breaks, the incentives for charitable giving have plenty of strings attached. To qualify, you must give at least 250 a year. Small donations, such as amounts given to street or door-to-door collectors, get nothing. Donations must generally be in cash, although the government has tweaked the rules by extending the tax relief to donations of stocks and shares from 2007.

Charities have welcomed the move although, because few Irish people dabble directly in the stock market, it is unlikely to make them rich. "Something like this will always be a slow burner in Ireland but we welcome anything that promotes philanthropy, " says Richard Dixon of the Irish Charities Tax Reform Group.

The group also wants the government to extend tax relief to other forms of non-cash contributions such as property. This could be a real money spinner judging by the early Christmas present that the St Vincent de Paul received from the late Maureen O'Connell. It is now 14m better off after the Galway woman left the charity the lion's share of her landmark pub on the city's Eyre Square.

"We're campaigning for tax relief to be allowed for a range of non-cash assets, " says Dixon. "In the States and UK giving shares is a popular way of supporting charities and we're looking for something similar to be introduced here."