From UO Evolution
This is the skill that allows a mage to function offensively. Without it, your spells will most of the time do less than 10 damage. Without Evaluate Intelligence, your magery skill will be nothing but heals, mind blasts, poisons and summons. Even bless and curse type spells rely on Evaluate Intelligence for their powers.
The damage bonus for evaluate intelligence can be determined through this formula: ((Evaluate Intelligence * 3) / 100) + 1.
For example, if you have 105.0 Evaluate Intelligence, your base damage would be increase by ((105 * 3) / 100) + 1 which would be 4.15. Essentially, your damage would be quadrupled, and that before any other damage source was added to your spells.
Similarly, the stat effect spells (Bless and Curse type spells) are determined solely by Evaluate Intelligence. They work like this:
Bless stat spells : (Your Evaluate Intelligence)/10 + 1 = Percentage Increase Curse stat spells : 8 + (Your Evaluate Intelligence)/10 - (Targets Resisting Spells)/10 = Percentage Reduction
A couple of examples to show how this works.
Example 1) You have 80.0 Evaluate Intelligence, and 90 Intelligence, and cast "Cunning" on yourself.
So, plotting in the numbers for the bless stat spells, we get: (80.0/10 + 1) = 9% Increase. We then apply that to the Intelligence score, and get: (90+9%) = 98(.1, but it rounds down). Your new temporary intelligence score is 98.
Example 2) You have 100.0 Evaluate Intelligence, and your opponent has 95 Strength and 90.0 Resisting Spells, and you cast "Weaken" on your opponent.
Once again, plotting in the numbers, this time for curse stat spells, we get: (8 + 100.0/10 - 90.0/10) = 9%. Then we apply those numbers to your opponent, and get: (95-9%) = 86(.45, but again, rounded down). Your opponent is now down to 86 strength temporarily.
The evaluate intelligence skill is primarily gained "passively" through spell casting. It is vitally important to understand that any spell in the magery spell book has the exact same chance of gaining evaluate as any other spell. There is no difference at all. As with the two other distinct mage skills, this skill does, however, require you to move around due to an anti-macro code on the passive gains. So, if you're solely looking for gains in evaluate intelligence, your best bet is most likely the 1st circle spell "magic arrow", as it only takes 1 sulphurous ash and 4 mana to cast. You may want to, if you're hunting, actively evaluate everything you combat, as it will provide a gain once in a while. That is really all there is to raising evaluate; this skill is very likely to cause you skill gain walls though, because there is simply nothing you can do to make it "harder" to succeed in a gain check.