I think you actually want the Keisei script, @jprspereira always mixes up those two 
It will show up on radical pages as well so you can directly see if it is also a kanji in WK. The main purpose of the script is however to show which kanji give you hints how to read them, so it is still possible that the script will just show “This radical is not considered a phonetic mark!” even if there is a kanji in WK (I don’t know if that actually happens).
Using the script is not so hard, you have to install the Tampermonkey browser extension/addon, and then click on the download link.
I don’t know if this was covered in the help pages, but also be careful when using Tampermonkey. It is not only useful for WK, you can find scripts for any webpage on sites like https://greasyfork.org, but you don’t want to just use any random script you find on the internet 
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When you install scripts, you are shown the script for a reason. At the very least look in the beginning that only @includes (places where the script is used) of the sites you expect are there, and not something like @include *:*.
Should be // @include https://www.wanikani.com/ or similar.