First since you are not the original owner your 3 year AMD Warranty is voided.
Does your APU have the following P/N tattooed on the processor's lid: https://www.cpu-world.com/CPUs/Zen/AMD-Ryzen%203%203200G.html
Here is AMD article on how to tell if it is genuine or not:https://www.amd.com/en/support/kb/warranty-information/pib-authenticity
Basically without the AMD Retail box it wil be difficult to verify. I suggest your open an AMD Support ticket and give them your APU's S/N and P/N for them to check if the Serial Number is valid or not.
Also compare the APU with photos from other same APUs to make sure they look the same. Sometimes a counterfeiter will replace the outside lid and glue it on a totally different but less expensive processor.