If you are testing the chemical composition of the anti-freeze in your 1999 Chevrolet Silverado it does not matter if it is hot or cold. If you are checking the coolant level it should be at the full cold mark when the engine is cold.
RepairPal Certified shops can diagnose and repair your 1999 Chevrolet Silverado 1500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.