How to check that only one value of my dictionary is filled?

How can I check that my dict contains only one value filled ?

I want to enter in my condition only if the value is the only one of my dict.

For now I have this if statement

my_dict = {}
my_dict["test1"] = ""
my_dict["test2"] = "example"
my_dict["test3"] = ""
my_dict["test4"] = ""
if my_dict["test2"] and not my_dict["test1"] and not my_dict["test3"] and not my_dict["test4"]:
   print("inside")

I would like to find a better, classy and "pep8" way to achieve that

Any ideas ?