You may be able to use the old steel pipe, if the resistance to ground is less than 25 ohms. Mike Holt has a good article that explains how to measure ground resistance. If the resistance to ground is greater than 25 ohms, you'll need a new ground electrode or additional ground electrodes.
National Electrical Code 2011
Article 250 Grounding and Bonding
250.56 Resistance of Rod, Pipe, and Plate Electrodes. A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.
National Electrical Code also provides a list of electrodes permitted for grounding (250.52(A)(1) through (8)).
- Metal Underground Water Pipe.
- Metal Frame of the Building or Structure.
- Concrete-Encased Electrode.
- Ground Ring.
- Rod and Pipe Electrodes.
- Other Listed Electrodes.
- Plate Electrodes.
- Other Local Metal Underground Systems or Structures.
If the Authority Having Jurisdiction (AHJ) does not determine that the old pipe is adequate for grounding, you can use any of the above methods to provide additional ground electrodes.
Fluke provides a good explanation of the Fall of potential test procedure.
Basically you disconnect the earth electrode from the system, then place two stakes in a line a specific distance from the earth electrode. A GEO Earth Ground Tester is connected to the stakes and the electrode, and then generates a known current between the outer stake and the electrode. The drop in voltage between the inner stake an the electrode is used to calculate the resistance of the earth electrode.
In both tests described in the Mike Holt article, expensive specialized meters are used to measure the electrode resistance. It's not likely a DIYer will be conducting this testing, so you'll have to relay on the AHJ to determine if an electrode is sufficient.