When a suspect test occurs, there will be an announcement posted to the subforum corresponding to that tier. You can check out that announcement for the instructions. Basically, you'll have to make a new account and ladder until you meet the requirements, and then post the evidence. Once the laddering period is over, you will be able to vote in a thread in the Blind Voting subforum.How do I start participating in suspect tests
You can find a link to a suspect test that is currently in progress here. Usually, suspect tests will have more or less the same instructions as this one (the recent Tera vote in SV OU being a notable exception).