Brand and reputation: choose well-known, well-reviewed manufacturers for better product and after-sales; learn reputation via online reviews and referrals. Performance report: ask for the system window's performance test report and check insulation, soundproofing, water/air tightness and wind resistance against national standards or your needs. On-site inspection: if possible, visit the plant or installed projects to see process, install quality and effect. After-sales: understand content and term — installation, commissioning, repair, maintenance; good after-sales solve problems promptly and extend service life.