When you can visit site A, but not site B on a same browser with a same device at a same time, then site B must be down or blocked in your area.
So, how to tell if a site is blocked or not? You can check out the following 3 ways:
1. To visit it with anti-censorship tools
If you can visit the site with VPN, SSH, Proxy or any other anti-censorship tools, but can't without any of them, then the site must be blocked in your area.
And if not, then the site must be down.
2. To ping it with command lines
For Windows, you can open the Command Prompt (cmd.exe), and for Mac OS, you can open the Terminal application, then enter the following command:
Remember to change the above "twitter.com" to the site you are going to ping.
If there are all "timeout" errors in the result, then the site is blocked in your area or its server is down, so how to figure it out?
You can run a VPN and ping the site again, if there are no or few errors, the site is blocked, and if there are still all "timeout" errors, then the site is down.
3. To test it with third-party apps
To check if a site is really down or just blocked, you can also check out IsUp.Me, Just Ping, Watch Mouse and some other free third-party testing web apps, with any of which, you can just enter the domain of any site and see if it is blocked or down.
To check out if a site is blocked in China, you can use the WebSitePulse service, which will tell you if a site is blocked in Shanghai, Beijing, Guangzhou or Hong Kong.