What is a primary purpose of the before takeoff checklist?

During the before takeoff runup, the alternator ammeter shows no output. To ensure the aircraft is in safe operating condition as recommended by the manufacturer, the pilot should

During the before-takeoff engine runup at 1,700 RPM in a typical single-engine airplane, you note the following: both magnetos selected—smooth operation at 1,700 RPM; left magneto—225 RPM drop with slight roughness; right magneto—150 RPM drop, smooth. Oil pressure reads 25 psi (green arc 30-55 psi). You are holding short at a busy airport and are number one for departure. What is the most appropriate action?

As part of the Before Takeoff Check to ensure the aircraft is in safe operating condition as recommended by the manufacturer, you note the following METAR for your airport: KORD 010653Z 36003KT 10SM SCT250 00/00 A2985 RMK AO2 Which condition indicated by this report requires specific inspection of the aircraft's exterior surfaces?