For client sites, do you check DNS or hosting first?

When a client reports that a website is down, the first step is usually figuring out whether the issue is with the domain setup or the hosting server. Sometimes DNS records or nameservers are the problem. Other times the server, SSL, CDN, or application is the real cause. For hosting providers, agencies, or domain resellers, do you follow a fixed checklist? Do you check DNS and nameservers first, or start with hosting logs and monitoring?