A taxpayer making donations to charity can claim tax deduction if:
It is a donation of money; and
It is for charitable purpose; and
It is paid to a charitable institution or a trust of a public character that is exempt from tax under Section 88 of IRO or to the Hong Kong SAR government; and
The total donations in the year of assessment is not less than $100.
A taxpayer cannot claim for the donation which has been allowed as a deduction in his spouse's Salaries Tax assessment or in his own Profits Tax assessment.
The maximum deduction is 25% of his net income (assessable income less deductions).
He may claim the deduction in tax return. If he forgets to do so, he may claim it by an objection when he receives the assessment. Even if he misses the objection deadline, he can still claim it by a Section 70A claim (because it is an omission --- to claim the deduction).
The Revenue's assessing practice on charitable donations are set out in Departmental Interpretation and Practice Note No. 37. You can read them on their homepage: click [English], [Publications], [Departmental interpretation and Practice Note].
Besides, you can check whether the charity is an approved one on their homepage : click [English], [Publications], [List of approved charities].