Certificate vs endorsement
The ADDL INSD box on an ACORD 25 is the agent's statement that an endorsement exists. The endorsement is the policy form that actually grants you coverage. On any job where a claim would hurt, ask for the endorsement pages and read them. Blanket endorsements (“any person or organization you are required by written contract to add”) are common and fine, as long as your subcontract contains that requirement.
The checklist
One: the endorsement names you, or contains blanket wording triggered by a written contract you actually signed. Two: it covers ongoing operations (CG 20 10, CG 20 33 or equivalent). Three: it covers completed operations (CG 20 37 or equivalent) — most construction claims arrive after the sub has left the site. Four: coverage is primary and non-contributory. Five: there is no limitation to the sub's sole negligence; you want coverage for claims “caused in whole or in part” by their work. Six: the auto policy carries the same additional-insured status if the sub drives on your site. Seven: the umbrella follows form and extends additional-insured status upward.
Common exclusions to look for
Residential or condominium exclusions, height exclusions above two or three stories, exterior insulation and finish system (EIFS) exclusions, subcontractor warranty endorsements that void coverage if the sub's own subs are uninsured, and “insured versus insured” exclusions that bar claims between additional insureds. Any of these can make a $2,000,000 certificate worth nothing for your project.
Making it routine
Put the endorsement requirement and the form numbers in your subcontract exhibit. Require the endorsement copy with the first certificate. Check the description of operations on every renewal for the same language. Track it as its own field, not as part of a general “certificate received” checkbox — it is the field most often wrong.