There are 3 leads in a transistor viz. collector,emitter and base. When a transistor is to be connected in a circuit,it is necessary to know which terminal is which.
There are three systems in general from which we can determine which terminal is which,they are as follows

1. When the leads of the transistor are in same plane and unevenly spaced then they are identified by the position and spacing of leads. The central lead is the base lead.the collector lead is identified by the larger spacing existing between it and the base lead. The remaining lead is the emitter.see fig i



2.when the leads of the transistor are in the same plane but evenly spaced,the central lead is the base.The lead identified by dot is the collector and the remaining lead is the emitter.see fig ii

3.When the leads of the transistor are spaced around the circumference of a circle,the three leads are generally in E-B-C order clockwise from a gap.see fig iii