The requirement level is defined by the item and every item has an internal level number.

Affix Level Minimum

In Diablo 3 every item has an internal level number, and every affix has an "affix level minimum". If an item's internal level number is below an affix's level minimum, the affix cannot be applied to the item. This prevents the incongruity of attaching extremely powerful affixes to crude, lowly items.

Source: Diablo 3 Signature Series Guide

There is no list with items and their internal level numbers so I guess they are assigned randomly for every item.

But at least legendary items also have a total affix count and bonus affixes. Here is an example:

Blackguard: LVL REQ: 48; TOTOAL AFFIX COUNT 6; BONUS AFFIXES: 2; GUARANTEED AFFIXES: Damage Bunus, Min/Max Damage, Strength/Dexterity, Crowd Control, Reduction

There is now an annoying and tedious way to do this. Now that you can see the stat differentials of chat linked items while in a game, all you have to do is:

• Shift+Click the item to copy it to the chat window
• Send it anywhere
• Start a game.
• Once you are in the game, you can click the item from chat and it should show you the differences.

If it does not, try sending the chat message again while in game. While this does require you to leave the Auction House, at least it gives you some way to compare those items. I recommend linking a bunch of items at once so you don't have to go in/out as often.