A Solid Foundation
Peer tutors don't have to have a perfect grasp of Spanish–after all, they are still students themselves. The "peer" part of peer tutoring includes modeling not just an advanced understanding of Spanish but also the learning process, with all its trips and stumbles. When peer tutors have to search for an answer to their student's question, they are actually building credibility–as a peer.
With that being said, peer tutors do need to steer the students they are helping toward proper answers, skills and techniques. While their understanding of the new language doesn't have to be perfect it should be consistent, and they should have a very good understanding of the fundamentals they may be called upon to teach or explain. Here are some of the clues that may demonstrate a student is ready to take on the job of Spanish peer tutoring:
Syllable Emphasis and Pronunciation
Spanish is a relatively easy language to pronounce. Each vowel makes only one sound, no matter where it is located in a word. Consonants are consistent too, as a rule, with c being the radical exception (two possible pronunciations instead of one).
Placing emphasis on the appropriate syllable is also a very simple process in Spanish, with regular rules and a clear indication (accent mark) whenever a word deviants from these rules.
Test your potential Spanish peer tutor's ability by having him read from a Spanish-language passage that is just past his current ability. Look for three things: Clear, consistent pronunciation, including stress on the appropriate syllables; the ability to pronounce a word correctly even if he does not understand its meaning; and, perhaps most important of all, the willingness to admit he does not understand a word as opposed to pretending or making something up.
If your student has a good grasp of the absolute basics and is willing to admit when he doesn't know the answer to a question (be sure to stress that this is OK–after all, he's still a student too, and anything he doesn't know is an opportunity for learning) then he may be ready to peer tutor other Spanish students. However, is he able to answer the inevitable string of "Why?" and "How?" questions that a beginner will bring to basic concepts he has already internalized? Test him by having him prepare and teach a review of one basic concept to the entire class. Watch how he presents the material–is it logically organized? Does he cover all the important concepts? Can he answer questions clearly and appropriately?
Open Channels of Communication
Make sure to stay in constant contact with both your peer tutor and his Spanish student or students. Personalities may or may not mesh, they might encounter a particularly difficult lesson or the student may even advance past the tutor's abilities. Make it clear that you are ready, willing and able to act as a resource for both students as they learn together. As teacher, you are still an integral part of the peer tutoring equation.
Finally, make sure your peer tutor is rewarded for his effort. This may include giving credit for peer tutoring in place of or in addition to credit for selected assignments. Keep careful watch to make sure that the extra work of Spanish peer tutoring doesn't become so much of a burden that it causes him to struggle in your class or others.