Assess the student first in the language spoken in your school and classroom. Walk the student though whatever language assessment you use on students who are not bilingual. There are many ways to do a language assessment, although in preschool you will probably use a combination of talking to the student and listening to the student talk with friends. Begin with this baseline assessment of whatever the primary classroom language is.
Perform the same evaluation in the child's second language. If you are bilingual, you can do this yourself. Otherwise, speak to your principal or administration about obtaining an aide that speaks the same language as the student does. The aide should perform the language assessment, again following whichever guidelines you use for your traditional preschool language assessment.
Compare the results of the two language assessments to determine if the child is at the same level in both languages. If the child is not at the same level in the classroom language as he is in the other language he speaks, he will need to have additional work in the classroom language so he can continue to develop language skills. It is not your job to teach him additional skills is if he not as fluent in his second language, unless your school is bilingual.