You don't seem to have a question about lavaan, but about R in general. I recommend posting your question on CrossValidated (stack exchange's stats corner). But you can use
complete.cases() on the subset of columns in your data that are part of your model, which will give you a logical vector of which rows were included after listwise deletion. Any FALSE values are the excluded ones.
Terrence D. Jorgensen
Assistant Professor, Methods and Statistics
Research Institute for Child Development and Education, the University of Amsterdam