In my old west r.p.g. characters who use magic, must make an Arcane Control check. This is a percentile roll modified by the caster's intellect modifier. At present I have only a succeed or fail outcome for this mechanic. Is this too basic? Should I add something for a player rolling a critical success (natural 01%) or a critical failure.(natural 100%)

For my own tastes, I might like results like:
easy casting - 1/2 cost/effort/time
hard casting - X2 cost/effort/time

Throw in an option to spend the normal amount on a easy cast and get X2 effect instead, and thrown in the option to drop a hard casting for only regular (or no) cost. I would suggest each result show up in the top or bottom 10-20% of the casters skill success range.

This gives you some sense of varied casting experiences, and some sense that the roll may offer your players a choice.

You say "Old West Game"...

The first thing that comes to my mind when you ask this question is...

Do native shamans and European sorceror types both have to use this "arcane control" check?

Quickly followed by...

What purpose does the check serve within the context of the game?

Is it simply a way to prevent magic being too powerful? Does it reflect a mystical unknown?

If you give us a bit more perspective, we can give you some more meaningful responses.

V

Both native shaman and European sorcerer types must make an arcane control check. The main reason for this rule is to represent that arcane energies are unpredictable. As a magic using character advances they learn to exert better control. However, I want to keep magic "wild", so it is never 100% controlled by the user.