I got an x-ray done and my two cavities filled. if there was a serious tooth infection like a dental abscess, would my dentist have noticed?

Possibly...: Firstly, the radiograph that you had taken would have had to show the tip of the tooth and the surrounding bone. Even if it did, a newly formed abscess does not always clearly manifest itself on a radiograph. Not everything shows up.

Definitely, yes: Dental abscess is easy to diagnose by dental/medical history, symptoms dental examination, and x-rays. Dentists are well trained to rule out dental abscess or other oral infection.

Yes: Your dentist should have picked up an abscess. If you are having symptoms have the dentist recheck your teeth.

Depends: Not all tooth infections are symptomatic. They can be pain free for years. An infection or an abcess can usually be seen on an x-ray it it shows the end of the root and the surrounding bone ( bitewing x-rays do not show the end of the root).