The pin numbers are often engraved in the plastic of the connector but 
 you may need a magnifying glass to read them. 
 Note DCD is sometimes labeled CD. 
 The numbering of the pins on a female connector is read from right to left, 
 starting with 1 in the upper right corner 
 (instead of 1 in the upper left corner for the male connector as shown below).
 --> direction is out of PC.

      ___________                    ________________________________________
      \1 2 3 4 5/  Looking at pins   \1  2  3  4  5  6  7  8  9  10 11 12 13/
       \6 7 8 9/  on male connector   \14 15 16 17 18 19 20 21 22 23 24 25/
        ------                         -----------------------------------
    Pin #   Pin #   Acronym  Full-Name      Direction  What-it-May-Do/Mean
    9-pin   25-pin
     3       2      TxD     Transmit Data       --> Transmits bytes out of PC
     2       3      RxD     Receive Data        <-- Receives bytes into PC
     7       4      RTS     Request To Send     --> RTS/CTS flow control
     8       5      CTS     Clear To Send       <-- RTS/CTS flow control
     6       6      DSR     Data Set Ready      <-- I'm ready to communicate
     4      20      DTR     Data Terminal Ready --> I'm ready to communicate
     1       8      DCD     Data Carrier Detect <-- Modem connected to another
     9      22      RI      Ring Indicator      <-- Telephone line ringing
     5       7      SG      Signal Ground

      9-Pin DB9 Connector                    25-Pin DB-25 Connector
    1     DCD   Carrier Detect             1           Chassis Ground             
    2     RxD   Receive Data               2     TxD   Transmit Data
    3     TxD   Transmit Data              3     RxD   Receive Data
    4     DTR   Data Terminal Ready        4     RTS   Request To Send
    5     SG    Signal Ground              5     CTS   Clear To Send
    6     DSR   Data Set Ready             6     DSR   Data Set Ready
    7     RTS   Request To Send            7     SG    Signal Ground
    8     CTS   Clear To Send              8     DCD   Carrier Detect
    9     RI    Ring Indicator            20     DTR   Data Terminal Ready
                                          22     RI    Ring Indicator