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Question: How do people know what they can get out of trade up contracts?
Answer: The Concept

The whole idea of trade-up's are pretty simple. It provides an alternative to simply opening cases or trading weapons with other players. In order to use the trade-up contracts, all you need is 10 weapons of the same grade.

Weapon Grades

The grades go in the following order:


  Consumer Grade > Industrial grade > Mil-spec > Restricted > Classified > Covert 


How it works?

Essentially, if you were to use 10 weapons of the Restricted quality, you will get 1 weapon of the Classified quality. If you were to use 10 weapons of Classified quality, you will get a single Covert weapon. Etc.

Here is an example:

Scrolling over an item in your inventory reveals a list. Something like this: 

This is called a "Collection". Basically, if I were to trade up 10 M4A1-S| Atomic Alloys, I would have a 50-50 chance of getting either a AK-47| Vulcan or a M4A4| Desert-Strike.

What about weapons of different collections?

Simple. Having more weapons of different collections in the same contract means that there are more different variations of weapons if you get. If I were to throw in 5 M4A1-S| Atomic Alloys and another 5 M4A4| Dragon Kings: 



I have a chance of getting the following weapons:


  
  AK-47| Vulcan
  M4A4| Desert-Strike
  AWP| Man-o'-War
  Galil AR| Chatterbox
  


To conclude,

Basically all you need to remember is that when you place weapons of lets say Collection A into a contract, you will get 1 of any weapon that belongs in Collection A. Just that it will be 1 grade "Better".

Extra:


Knives, which have the grade Exceedingly Rare ★, CANNOT be used in contracts
You can now use Stat-Trak weapons in Trade-ups
I am not entirely sure, but I believe that "Wild-Card" skins, are essentially that one or two unwanted skins that have been placed in the contract just so as to fill it up. Since there is only 1 or 2 of them there is a low chance of their collection being picked, but still, it is possible. Hence they are "affectionately" referred to as "Wild-Card" skins.