Get a digital level (I use a Sears version) and build a bracket (mine is made of scrap aluminum) that can be temporarily attached to the prop blade to hold the digital level. Be sure to position the bracket/level on each blade in the same relative location to facilitate the blade pitch comparison. There is much discussion regarding this matter in the VAF/RV-12 blog.
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B) - N124DH flying since March 2014 - 700+ hours (as of Nov 2018)!
through June 2019.