1. Consult a repair manual: A Haynes or Chilton manual for your specific year and engine will have a diagram showing the firing order and the distributor cap terminal layout.
2. Look for markings: Carefully examine the distributor cap and rotor. There might be faint markings indicating cylinder number 1.
3. Use the firing order: The firing order for a Chevy S10 (usually a 2.5L or 4.3L V6) is commonly found in repair manuals or online. Knowing the firing order allows you to trace the spark plug wires and find the #1 cylinder. Then, follow the wire back to the distributor cap to find the terminal (and thus the rotor position) corresponding to cylinder #1.
Do not rely solely on general information. The exact location can vary depending on the specific distributor installed in your vehicle. Using a repair manual is the safest and most accurate method.