Take a look at my most recent post for tips on how to prepare ELLs for language testing.
Assessing students language level is an ongoing process. My county uses WIDA assessments when students enter the county and in the winter for a formal language level. Throughout the year we use formative assessments to determine students language progression.
This past year I developed a number of formative assessments to help me with this. The assessments are linked to science or social studies topics when possible. There are separate primary and upper grade level assessments. I found them helpful when I was working with a large group of students and want to get a picture what they can do without the usual assistance they are given on typical assignments. These assessments are designed to help prepare them for the academic language they will encounter when taking standardized tests such as the WIDA ACCESS assessment.