I have a loop within a loop: looping through sub-pages and arrays contained within. Within the inner loop, I'm checking to see if object an position has a bool set to TRUE.

		<% loop $Menu(1) %>
						<% if ClassName = Gallery %>
							<% loop $Children %>
								<% if $isHero %>
									
									THE POS:
									$Pos

									<div class="item">
										<div class="center">
											<a href="$URL"><img  src="$Photograph.Link"  class="full-width img-responsive" alt="" /></a>
										</div>
									</div>

								<% end_if %>
							<% end_loop %>
						<% end_if %>
					<% end_loop %>

So, I only output HTML if the inner condition is met. However, I also want to add a class -- .active -- to the first DIV but as the object is not necessarily the first in its array, I can't use $First or $Pos.

Is there any way to check the iteration of the loop instead of the index of the object?

I solved the problem without needing the iteration number:

<div class="active item">
					<% loop $Menu(1) %>
						<% if ClassName = Gallery %>
							<% loop $Children %>
								<% if $isHero %>

										<div class="center">
											<a href="$URL"><img  src="$Photograph.Link"  class="full-width img-responsive" alt="01" /></a>
										</div>
									</div>

									<% if $Last %>
									<% else %>
									<div class="item">
									<% end_if %>

								<% end_if %>
							<% end_loop %>
						<% end_if %>
					<% end_if %

As my first div always has the .active class, I open the div outside of the loop. Then I use a conditional to check whether the item is $Last. If it isn't, I open a new div without the .active class.

Although you've solved this another way, the answer to the original question (Thread title - "Getting iteration number of loop") is $Pos

Also for syntactic sugar, since 3.0 (or there abouts) you do not need an empty if clause to get an else. (<% if $Last %><% else %>output<% end_if %>)
You can simply use the keyword 'not': <% if not $Last %>output<% end_if %>