so first i recommend you download a backup copy of your template or use a test blog.
the reason i did this is so that we can easy identify it when we switch to the edit html tab make sure you have the expand widget templates checkbox checked and find the code for your widget here's a screenshot of mine
Now once you have found the widget code you need to decide which page you want it to be displayed on.
Here's the code for displaying widgets on the postpages
<b:if cond='data:blog.pageType == "item"'>
Here's the code for displaying widgets on the homepage
<b:if cond='data:blog.url == data:blog.homepageUrl'>
Here's the code for displaying widgets on the archive pages
<b:if cond='data:blog.pageType == "archive"'>
now add it to the widget like i've done in the following screenshot
save and preview your template to see it in action
Now if you use the AdSense widget in your blog posts by default it will show them on the main page with a little tweak we can get them to show on the post pages only by moving the closing if tag further down here's a screenshot of the post widget before and after
Now if you make the above changes to the post widget and save it AdSense will only be displayed on each post page. I hope you enjoyed reading this post dont forget to comment if this works for you