If you need to know which CPU a process is using, top can be used to find out.

 1. start top
 2. hit the f key
 3. you can now select the fields you want to display. 
    P gives the last CPU used so select that.

You will now see output along the lines of:

 PID  USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  P SWAP COMMAND
20394 oracle    15   0 20.2g 133m 129m S  6.6  0.3   0:37.88 3  20g oracle
24934 oracle    16   0 20.2g 185m 181m S  5.3  0.5   1:11.39 3  19g oracle
18065 root      18   0  288m  14m 9360 S  3.6  0.0  69:18.57 2 274m beremote
22802 oracle    18   0 20.2g 1.5g 1.5g D  3.6  3.8   0:03.67 1  18g oracle
10398 oracle    15   0 1226m  39m  11m S  2.0  0.1 297:38.05 0 1.2g java

The P column shows the CPU the process is using.