When moving web hosts you are going to have some downtime unless you are using the name-servers of your registrar, in that case you just need to change the ip address of your new host/VPS server. Below is a list of my favorite DNS tools:
Open DNS Cache check
This tool from OpenDNS helps you check what server OpenDNS customers see when they request a domain. It includes their servers across the world including North America, Europe and Asia.
Pingdom DNS tool
I highly recommend the tools from Pingdom including their DNS server checker. Just enter your domain to check if its DNS servers are configured correctly. After the test it gives you aÂ Permalink to allow you to share the test link. The results are either in Basic view mode for easy reading or Advanced mode for you techies out there.