Ad Hominem occurs when you attack your opponent instead of their argument. This can take multiple forms:
Each of these Ad Hominem types attack the person instead of their argument. A simple check to do is consider if your argument would work if you had a different opponent. If your argument is dependent on your opponent it is likely an Ad Hominem Fallacy.
Point out that your opponent is now attacking you or someone else instead of the actual argument. Restate the original argument and ask them to address it directly.