Here is a rationale for evaluation and teaching of spelling by D.K. Reed (2012), funded by the US Department of Education. It includes a chart of (basic) spelling expectations by grade according to the Common Core State Standards.
1. Collect a free writing sample from the student. A 15 minute sample should provide ample material for analysis.
2. Make a photocopy of the writing sample that you will mark on.
3. Decide what elements you want to analyze. This will vary for students of different ages, but for all students it is helpful to calculate the total number of words written and the percentage of word spelled correctly.
If the writing sample was timed (e.g., 15 minutes) calculate the words written per minute as a measure of writing fluency.
For young students (e.g., grades 1 - 2) you may want to count things like:
the percentage of closed syllables ("short vowels) that are spelled correctly
the percentage of one syllable words spelled correctly
the percentage of word-initial consonants spelled correctly
the percentage of word-final consonants spelled correctly
For older students (grade 3 and older) you may want to count things like:
the percentage of vowel team syllables that are spelled correctly
the percentage of multi-syllable words spelled correctly
the percentage of words with suffixes that are spelled correctly
CALCULATE PERCENT CORRECT
To calculate percent accuracy of any specific element, count the total instances of the element in the sample (e.g., the total number of vowel team syllables) and then count the total number that were spelled correctly (e.g., the number of vowel team syllables spelled correctly) and then divide the number correct by the total number. (total correct / total number = % correct)
You may also choose to analyze the types of spelling errors the student made. See, for example, the chart in this article: Analyzing Spelling Errors.
A spelling analysis can serve both to inform intervention and as a progress monitoring tool.