Watch the vehicle ahead pass some fixed point - an overpass, sign, fence corner, or other marker.
Count off the seconds it takes you to reach the same spot in the road ("one thousand and one, one thousand and two...").
If you reach the mark before you finish counting, you are following too closely. Slow down and check your following distance again.
The two-second rule applies to any speed in good weather and road conditions. If road or weather conditions are not good, double your following distance. You should also double your following distance when driving a motor home or towing a trailer.
Following Distance For Trucks
A truck or any vehicle towing another vehicle may not follow within 300 feet of another truck or vehicle towing a vehicle. This law does not apply to overtaking and passing, and it does not apply within cities or towns.