I usually work with a single type of cell (in my case B-cell lymphocytes). In my case I want to determine if a cell is dead, dying or healthy. I use dyes and proteins against markers on the cell to figure out which state thety are in. I can use the microscope to look at the cells directly, or I can use a flow cytometer to get a rough idea of the level of marker on each of thousands of cells.
If you have a mixed population (like blood or cells extracted from a tumour), you can separate them based on their size, how much light they scatter (that’s a rough measure of internal complexity), or you can attach panels of labelled antibodies to the cells. The antibodies bind to certain markers on the cell surface, if a cell has some or all or none of a collection of markers that can help you to figure out what type of cells you are looking at.