Proxy protocol checkeris it HTTP, HTTPS, SOCKS4 or SOCKS5?

Paste it above and this tool will tell you. It probes HTTP, HTTPS, SOCKS4 and SOCKS5 separately, with a real handshake for each, and reports which ones answer. So a proxy handed to you with no protocol, or the wrong one, shows its true protocols here.

You test it, you cannot tell from the address. The port is a hint people rely on and it is often wrong. This checker opens each protocol's own handshake and reads which succeed, so the answer comes from the proxy itself rather than a convention.

Yes. Plenty of proxies answer on more than one protocol, and HTTP and HTTPS in particular are almost always the same proxy. This page checks all four independently, so it shows every protocol a given address actually speaks instead of picking one.

An HTTP proxy understands web requests and carries web traffic. A SOCKS proxy is a lower-level tunnel that carries any TCP connection, and SOCKS5 can also do remote DNS and UDP. For plain scraping, HTTP is simple; for anything that is not a browser, SOCKS5 is the more capable tunnel.

Because they are genuinely different services on the same address. A proxy might forward plain HTTP but refuse the CONNECT that HTTPS needs, or speak SOCKS5 but not HTTP at all. Testing each protocol on its own, which is what this does, is the only way to see which half works.

It can. An HTTP proxy is the easiest to give you away, because it can add headers that carry your real IP; SOCKS proxies do not add those. This checker grades every proxy elite, anonymous or transparent from what it actually forwards, so you see the real answer per address rather than a rule of thumb.