Dissertations tend to be labeled as a dissertation on the front page of the document. Look for mention of a faculty mentor, dissertation committee, mention of degree completion, etc. Dissertations also tend to be quite a bit longer than the typical journal article.
For example, in EBSCO databases, an icon on the results page next to results indicate certain results as dissertations:
In Google Scholar, dissertations are often pulled from the ProQuest database and can be identified from their preview:
You may also find dissertations in Google Scholar from institutional repositories from universities: