How can I say if a URL is a post, page, or taxonomy archive?

WordPress sites usually use body_class(), which adds CSS classes to the <body> tag. If you inspect the body element itself, most sites will show you something like

<body class="page-template page-template-tpl-events page-template-tpl-events-php page page-id-1168 page-parent page-child parent-pageid-378">

You can tell from these classes that this is a Page, using a Page Template, it is a Parent, and it is also a Child. You’ll see similar classes that tell you when you’re on a Post, Archive, etc.