How to implement validation of field values? I'll soon be involved in a project that requires this, I haven't started yet. I have some experience with Drupal but I'm not that much of a programmer.

Here's how it goes:


Create a content type with field 1,2,3.
The user A adds a new node, filling those fields.
The user B can go through those fields and do some kind of "OK" in each one. This OK will only trigger something else that will show a cross or a check mark instead, it won't have any interference with the field, which will not be changed.
The user A1 adds a new node, and the user B has to go all the way again.


I'm asking your opinion on possible options to accomplish this.
I understand that maybe I'll have to write some code...

Here's the draw that illustrates this:
You could use the Flag module for this. Excerpt from its project page (bold markup added here):


  ... Flag is a flexible flagging system that is completely customizable by the administrator. Using this module, the site administrator can provide any number of flags for nodes, comments, users, and any other type of entity. Some possibilities include bookmarks, marking important, friends, or flag as offensive. With extensive views integration, you can create custom lists of popular content or keep tabs on important content.
  
  Flags may be per-user, meaning that each user can mark an item individually, or global, meaning that the item is either marked or it is not marked, and any user who changes that changes it for everyone.
  
  In this way, additional flags (similar to published and sticky) can be put on nodes, or other items, and dealt with by the system however the administration likes.


If you're not familiar with the Flag module, checkout the set of 8 video tutorials about the flag module. Refer to Flag's Community documentation for (way) more details.

Specific to your 3 fields here, you'd need 2 flags for each field X (x=1, 2, 3) (so 3*2=6 flags in total) whereas you'd add these flags to the nodes where these fields are used:


1 flag to indicate Field X is Valid.
1 flag to indicate Field X is Not Valid.


And if you'd then use either of the options I described in my answer to "How to add a custom icon or symbol to a flag link?", you can even make its look and feel match with your requirements.

Optionally also using some CSS magic to align 2 of these flags next to the field X that they are related to (as suggested in a comment below this answer, merci!).

But wait, there is even more: by using all sorts of integrations with the Rules module, you could also implement things like:


"Send an eMail" to User B whenever User A does something that waits approval/review from User B.
"Send an eMail" to User A about the result (approve or reject) from User B.
Implement any kind of validations between those fields, such as "If field 1 has value P, then field 3 is not allowed to have a value of Z".


There are tons of other enhancements you could implement (potentially even causing User B to no longer be needed, since Rules becomes the validation engine ...).