is it possible to visualize coronavirus infection and what type of assays are routinely used?
If you have an antibody against the viral proteins you can do an immuno staining followed by fluorescence imaging. For this type of technique you would need a very specific antibody. I am not aware that this exist at the moment and I don't think anyone had the time to do it for immunostaining purposes.

For FISH it would depend on the genomic nature of the virus (ssRNA, ssDNA, dsRNA, dsDNA), again you would need specific probes to capture it. Again specific is key.

EDIT 01.04.2020: This group says they have found specific antibodies https://www.centivax.com/covid19