The right to self-determination of all peoples was confirmed by the United Nations General Assembly (GA) in the Declaration of Friendly Relations, which was unanimously adopted in 1970 and is considered an authoritative indication of customary international law.
Criteria for the right to self-determination 自決權的標準
A people can be said to have realised its right to self-determination when they have either (1) established a sovereign and independent state; (2) freely associated with another state or (3) integrated with another state after freely having expressed their will to do so. The definition of realisation of self-determination was confirmed in the Declaration of Friendly Relations.
In particular, the use of force to prevent a people from exercising their right of self-determination is regarded as illegal and has been consistently condemned by the international community. The obligations flowing from the principle of self-determination have been recognised as erga omnes, namely existing towards the international community as a whole.
~diakonia.se INTERNATIONAL LAW AND SELF-DETERMINATION『國際法及自決』