Key for determining if a post is on an index page, or displayed on its own?

On pages where multiple posts are displayed, the <h1> is the page name, e.g.: Archives, Home. Posts titled are rendered in <h2>. On a single post page, the post title is an <h1> tag and subheadings are <h2>, <h3>, etc as needed. All good so far.

The issue arises when a post with subheadings is not displayed on its own page, e.g.: when shown on a page showing the ten most recent posts in full. There, the titles are rendered in <h2>, but then the subheadings are one level off.

My thought was to write the headings within each post something like: {{{#singleentry}}}h2{{{/singleentry}}} {{{#multientries}}}h3{{{/multientries}}}

But I am not sure what the appropriate key would be, if it exists. I had a look through the developer reference doc but didn't see something like this.

Since something like this would be useful for things beyond just modifying heading levels, I feel like there must be something that will do the trick that I am simply not seeing.

If there's a better way, I'm all ears!