There are many types of transistors in the market, most of the time by reading the first number or alphabet printed on the transistor we can know as which type the transistor belong to.
A or 2SA is the PNP high frequency
B or 2SB is the PNP low frequency
C or 2Sc is the NPN high frequency
D or 2SD is the NPN low frequency
2SE is the P-gate thyristor
2SG is the N-gate thyristor
2SH is the N-gate UJT
J or 2 SJ is the P-channel FET
K or 2SK is the N-channel FET
3SK is the Dual gate FET
2SM is the Triac
These are just some of the standard part number for transistors. However there are also manufacturers that use their own part number like the International Rectifier Semiconductor that prefer to use the code IRFXXXX (for p and n channel fet), Philips is using the BUXXXX series for representing the horizontal output transistors. Sometimes it is quite confusing to memorize all sort of part numbers in transistors but as time goes by and you have been exposed many time in electronic repair, i believe you can identify a transistor part number in no time. At first, just search through the semiconductor databook and internet for the part number information.