Vetting a supplier before the first order
Check the supplier's commercial registration, ask for references from projects of a similar scale to yours, and request product certifications or test reports for any structural or safety-critical material.
Quality assurance during delivery
Inspect a sample from every batch against the agreed specification before it's accepted on site, and keep photographic records of deliveries in case a dispute arises later in the project.
For large or recurring orders, consider a third-party inspection for the first few shipments until the supplier has proven consistent quality.
Protecting multi-phase projects
Lock in pricing and specifications in writing for each phase of the project, not just the first delivery, so a supplier can't quietly substitute a lower-grade material once the relationship is established.