Look at certifications: Try to look for machining suppliers with business certifications like ISO 9001
http://dimovaa.com/index.php/Mastering_the_Artwork_of_Precision:_How_CNC_Precision_Machining_Revolutionizes_Industries_22068
Test certifications: Hunt for machining suppliers with marketplace certifications like ISO 9001, which ensures good quality management units are in place