There is no simple solution to your question. If it was a group of 4 students you could probably determine their level fairly quickly just by having a conversation with each one or observing them doing some task. The whole "what level is the student" is not a perfect science, although you could certainly find a list a linguist made that details what language skills the student should possess to be at a certain level and categorize them accordingly. I don't know many teachers that do this, usually there are simple cut offs like, is the student conversational, can they perform basic tasks and understand directions in the target language, etc. If we are talking about a classroom of 20-35+ students (the norm in Korean public schools) then there is just too much disparity in the students ability and you will have to get to know your students over-time and eventually you will be able to decide what they can handle and what they can't handle. I think the best thing to strive for is making groups that are of mixed ability and letting the high level students assist the low level students, but not do the work for them.