You may try iptables with the -v option - it will show you the number of packets and bytes that have so far matched a specific rule, or the chains default action (as your INPUT DROP action).
Also, fire up "tcpdump port 80" on your Raspberry Pi. It will show you all packets on port 80, even before iptables may drop them. Another possibility would be "tcpflow -c port 80", it shows you less info about packets and more about the actual HTTP stream.
Both tcpdump and tcpflow are usually not installed by default, so you'll have to do that first.