fouled plugs

Hi all, i recently had a set of 906 heads reworked and since installing i noticed my plugs on my pass side are fouling very badly. Before i had the heads done i didnt notice the fouling but i honestly cant really remember. my question to you all is...How can i verify what is causing the fouling? i am guessing a leakdown test, going to order it this week. I already did a compression test and all cylinders are between 170 and 175 psi.

Forgot to add this is a 383