Absolute error is the positive difference between the estimated value minus the actual value. A 25 mL burette will have different volume markings. What you can do is make different volume measurements. About three would be sufficient. You can measure 25, 15 and 5 mL.
Now, that will be the estimated volume value. To determine what the actual value is, what you can do is weigh the samples taken with a properly calibrated balance. Let the balance have one or two decimal places. With the density of the substance you use and the mass of the balance you can calculate the actual volume.
Then the procedure would be as follows:
1. Measure a certain amount of volume with the burette, e.g. 25 mL. You can use water. This will be the estimated volume.
2. Weigh the empty burette and then weigh it with the measured water content. The difference will be the weight of water with a volume of 25 mL.
3. Now you use the density of the water, at the temperature and pressure you are working at.
4. Using the density formula, you make the change from mass to volume. This will be the actual volume
5. Make the difference between the estimated volume minus the actual volume. This will be the absolute error.