A spell’s power often depends on its caster level, which for most spellcasting characters is equal to her class level in the class she’s using to cast the spell.You can cast a spell at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the same caster level.
In the event that a class feature or other special ability provides an adjustment to your caster level, that adjustment applies not only to effects based on caster level (such as range, duration, and damage dealt), but also to your caster level check to overcome your target’s spell resistance and to the caster level used in dispel checks (both the dispel check and the DC of the check).
It is possible for the Archmage to cast his Magic Missile with a caster level of 2 and only shooting 1 missile.
Be prepared to come up with a reason why you're going easy on the Magic Missile.... and why you aren't wearing the items he normally wears...
Most people are not going to know enough about magic to tell the difference. Unless the person has spellcraft they will not be able to identify a spell, much less figure out how high level the caster is. Spellcraft is also a trained only skill so unless the person seeing him cast actually put ranks into it they do not get a roll. The average commoner is not going to be able to tell the difference.
Most spell casters will at least get a roll, but there could be many reason why he only shoots a single missile. There are also ways to disguise it. The easiest way would be to carry a fake wand. Then simply pretend you are using a wand instead of casting a spell.