establish installed dependency not in portfile

[db]

I updated my ports and realised that gcc7 was built, but couldn't establish for which port it was necessary. I thought something like `port cat installed | grep '^name\|gcc7'` would help reveal it. Then I tried `port rdeps installed | grep ' ports\|gcc7'` and noticed py27-gnuplot, py27-numpy and scapy needed it, but by skimming through their respective files I couldn't establish the dependency.

How can I check if a dependency that requires such long build time is required beforehand?