Different ways to determine order of a reaction from:

(i) Rate law- The sum of powers of all the conc. terms is equal to the order of the reaction. It May be zero or even fractional. 

(ii) Unit of K- For zero order reaction unit of K is mol L^-1 sec^-1 , for 1st order reaction unit of K is sec^-1, for 2nd order reaction unit of K is mol^-1 L sec^-1  

(iii) Initial rate method- A chemical reaction is carried out with different initial conc. of each reactant and rate of reaction is determined in each case. The rate law is written in each Case. Rate laws are divided suitably to get the value of powers raised to each conc. terms in rate law. 

(iv) Integrated equation- In this method integrated rate equations for zero, first & second order reaction are used for given data. If the value of K comes to be constant, putting different time& concentration, using a particular expression so the reaction follow that order. 

(v) Half life period- Knowing the time required for the completion of 50% of the reaction we can identify the order of the reaction. t_1/2 for zero order reaction is directly proportional to the initial conc. of the reactant, t_1/2 for 1st order reaction is independent of initial conc. of the reactant. 

(vi) From reaction mechanism- For a complex reaction different steps are written. (Or from given reaction mechanism). The slowest step is identified; the order of over all reaction is equal to the no. of conc. terms involved in the slowest step.