If I don’t see ads, I assume Pihole is working. However I’ve been having issues lately and after tweaking settings I’ve been using various websites which tell you if your ad blacker is working. Like:
Do these sites tell you if your pihole is working? The first one tells me yes, the second one just doesn’t show any ads, and the third one says I’m not blocking ads. Just curious if this works for you. Especially the third one.
Many of the "ad block testers" check for HTTP level ad blocking and not DNS-based blockers like Pi-hole. So many of those sites will provide false positives. [Link removed, no longer available]
You can see real-life examples of Pi-hole at work. You can also view the Pi-hole log with pihole -t or via the Web interface to see the domains (the ads) being Pi-holed.