You can get tested to check if you ever had chickenpox. Before my chickenpox vax, the doctor tested to confirm that I never had chickenpox.
Varicella-Zoster Virus (VZV) Antibody Test (IgG)
This test detects IgG antibodies, which indicate past infection or vaccination.
A positive result means the person is immune (either from having had chickenpox or from the vaccine).
A negative result suggests no prior infection or vaccination, meaning the person is susceptible to chickenpox.