The "nslookup" command is your friend.
If you use nslookup on a client PC (runs on Windows as well as Linux) , local caching on a client wont trouble you:
nslookup pi.hole
Or a naughty domain:
nslookup doubleclick.com
Both should resolve (the Name address field) to the Pi-hole IP address.
You can use the nslookup command to query any DNS server like for example the router IP or Google's DNS server 8.8.8.8:
nslookup doubleclick.com 8.8.8.8
Compared to:
nslookup doubleclick.com <PIHOLE_IP_ADDRESS>