If you look in the OPF document, you will see in the meta properties, rendition:layout for a fixed layout document. In theory, it has two possible values, pre-paginated for fixed layout and reflowable. Though I've never seen reflowable used since that is the default. In the same area will be other meta properties such as page size in pixels, default viewing mode, etc. Some of these values can be specified in the <head>...</head> for individual files.
And then you have the mess inside each xhtml file where the positioning of everything is done. This can vary from images to individual letters. See the sample below for locating words that are part of a sentence.
Code:
<meta property="rendition:layout">pre-paginated</meta>
<meta name="fixed-layout" content="true"/>
<meta name="original-resolution" content="1216x1849"/>
<meta name="orientation-lock" content="none"/>
<meta property="rendition:orientation">portrait</meta>
Code:
<div class="liw c44 c45 c46 c47 c48 c17A c6F c7 c70 cAC cAD cAE cAF cB0 c17D" style="top:1382px">
<span class="w c76 c7F f22 c71"> holds </span><span class="w c18 f22 c71">a </span><span class="w cA5 c81 f22 c71">bag </span><span class="w c7F f22 c71" style="width:46px">of </span><span class="w cA3 c89 f22 c71">what </span><span class="w c7F c23 f22 c71">look </span><span class="w c21 c11F c18A f22 c71">like</span>
</div>
On one occasion, I converted a fixed layout to a reflowable and will say never again. It was a total pain much like converting a PDF to ePub