These D100's are HUGE - 45mm and much easier to read than it seems. The roll is the number on the flat side facing straight up and once you see it, you'll be able to easily find the number each time.
A 100 sided die, or D100, invented by Lou Zocci in 1985 is sometimes called Zocchihedron or "Zocchi's Golfball". Rather than being a polyhedron, it is more like a ball with 100 flattened planes and is designed to handle percentage rolls in games, particularly in role-playing games like D&D where it is used popularly for "Wild Magic" or "Wand of Wonders".
