How can you tell inside of a template if the page is being viewed as a single entry page or not?
You would use the {total_results}
variable (link) inside of the {exp:channel:entries}
loop that generates the entries that you are targeting. For instance, if you're displaying entries for a blog, your template code could look something like this:
<ul class="entries">
{exp:channel:entries channel="blog"}
{if "{total_results}"=="1"}
<li class="single-entry">
{title}
{content}
</li>
{if:else}
<li class="entry">
<a href="#">{title}</a>
<p>{excerpt}</p>
</li>
{/if}
{if no_results}
<li class="no-entries">No blog entries found.</li>
{/if}
{/exp:channel:entries}
</ul>