An assay is an analytic tool often used in a laboratory setting in order to assess or measure some quality of a target entity.  In virology, assays can be used to differentiate between transformed and non-transformed cells.  Varying the assay used, changes the selective pressure on the cells and therefore can change what properties are selected in the transformed cells.
Three common assays used are the focus forming assay, the Anchorage independent growth assay, and the reduced serum assay.
The focus forming assay (FFA) is used to grow cells containing a transforming oncogene on a monolayer of non-transformed cells.  The transformed cells will form raised, dense spots on the sample as they grow without contact inhibition.  This assay is highly sensitive compared to other assays used for viral analysis, such as the yield reduction assay.
An example of the Anchorage independent growth assay is the soft agar assay.  The assay is assessing the cells' ability to grow in a gel or viscous fluid.  Transformed cells can grow in this environment and are considered anchorage independent.  Cells that can only grow when attached to a solid surface are anchorage dependent untransformed cells.  This assay is considered one of the most stringent for detection of malignant transformation
In a reduced serum assay, cells are assayed by exploiting the changes in cell serum requirements.  Non-transformed cells require at least a 5% serum medium in order to grow; however, transformed cells can grow in an environment with significantly less serum.