In an effort to avoid the complexity and vagueness of MIL-STD references, some manufacturers provide IP (Ingress Protection) ratings for their equipment. IP ratings consist of a single number, such as IP44, and classify the degree of protection provided for electrical equipment by enclosures. The first number indicates, on a scale from 0 to 6, the protection against solid objects penetrating the housing. The second number gives, on a scale from 0 to 8, protection against penetration by liquids. For both numbers, a 0 means no protection and the highest number maximum protection. A rugged PC may, for example, have an IP54 rating. The "5" means that the device offers complete protection against contact with live and moving parts and also offers protection from dust deposits. The "4" indicates protection against splashing from any direction.
The National Electrical Manufacturers Association (NEMA) provides yet another classification system. NEMA 250 describes a variety of different enclosures and how they hold up against different environmental impact.
I hope this will get you started in determining the ruggedness requirements for your rugged PC. It probably also gives you an idea of how difficult it is to weigh ruggedness against other important factors, such as size, weight, performance and price. Even the laws of physics come into play: it is easier to protect a small, light device than a large, heavy one. Smaller screens are less likely to break, and solid state storage is inherently less prone to failure than rotating media. A display with a thick protection shield in front of it may be difficult to read, and a completely sealed keyboard hard to type on. Finally, use your judgment. I've seen supposedly rugged machinery that didn't look like it would live up to the task and some plain vanilla consumer products that felt bulletproof.
Rugged computers are a bit like off-road vehicles where the spectrum ranges from fun "lifestyle" models such as the Honda CR-V all the way to the mighty Hummer. Only you can decide which rugged PC is right for you.