Unique element to identify posts

Hi!

I work a lot with Google Tag Manager. I am programming a serie of tags to track user behaviour (time to read a post, type of user taking into consideration the previous metric and the number of words of a post (scanner, reader, intensiv reader…), identify users that comment, that share content in social networks…)

I would like to fire these tags only in posts (not in pages or other elements). I could use the URL as a firing rule, but I should update those rules every time I add new categories. As you know the whole code better than me, maybe you can help me saying of the following approach should be ok:

The id=”top” is unique and is always in the element body.
The class “single …” is used only in posts
body always have an id top and its corresponding class

If those arguments are true, then I can use the class of the body element to identify posts and send the Google Tag Manager Tags (document.getElementById(“top”).className;)

Thank you!!

Agustín