Determining if Layer is Point using Python Toolbox?

I think it may be as simple as changing this line:

describe = arcpy.Describe(parameters[0].value)

or maybe

describe = arcpy.Describe(parameters[0].valueAsText)

In his comment, @ian is correct about AddWarning() being useless outside of a tool's execute() method. I think the method you do want is setWarningMessage(). Try this:

describe = arcpy.Describe(parameters[0].value)
if describe.shapeType in ('Point', 'Multipoint'):  # catch point & multipoint
    parameters[0].setWarningMessage('This is a point feature class')
else:
    parameters[0].clearMessage()