See Figure 1. Sportster models with the ABS option can be identified by a wheel speed sensor on the left side of the front wheel. The wheel speed sensor has a clip with 'ABS' on it.
See Figure 2. These motorcycles also have an ABS module (EHCU) mounted just in front of the rear wheel.
Figure 1. Wheel Speed Sensor Clip (ABS Identification)
Figure 2. ABS Module (EHCU)