Where can I show colleges that I got perfect score on an AP exam?

When you login into the Common App, you first go to the Common App tab. Then click on Testing. Under Testing, you will see a prompt that asks "Indicate all tests you wish to report. Be sure to include tests you expect to take in addition to tests you have already taken." from the list click on "AP Subject Tests". That automatically creates an "AP Subject Tests" where you indicate how many AP tests you have taken and then you supply the Test Date, Subject Type, and Score for each of the tests you've taken.

This is how admissions officers reading your file see your what standardized tests you've taken and what the scores are.

Whenever you take an ACT, SAT, AP, or IB test and get a great score, it's important to indicate that on your Common App or Coalition App. Since most college allows you to self-report on your college application, you can use the Common or Coalition app prompts and fill them out yourself. However if you get admitted, you are obligated to send them an official report from the testing agency. Other colleges expect you to submit your test scores when you apply.

Good luck.

Yes, a perfect score on an AP is very rare - mention it in your honors with the statistic of how many other students got it (eg “Perfect score on X AP (only 2 students out of 25,100 achieved this distinction).”)

I don't think you know whether or not you know if you got a perfect score. Getting a five is not getting 100% correct. Colleges will see your scores when you send them. They will take into account how you did. Or you could write an essay on how hard you worked toward getting a 5 on the AP test.