Certain preferences,  physical traits came from his father. The blond hair, the skin.  Other things were taught.

But other things, habits, behavior Eobard didn't expect were passed down to Thad. Things he didn't realize himself. Besides the speed force connection.
 

There both disasters when it comes to dating\flirting\ people they like... Thad isn't equally awkward,  he's worse. Having no clue what to do and being side tracked.

Smiles. When Thad actually have a good reason to you, you can see part of Nora right there.. Not the typical one from inflicting pain, something innocent. Good, one of the clues that tipped off the connection.

The desire to have the one parent you never truly knew around, and the desperation to be around them. Thad would've done anything for her.  And Nora did the same for her father. Willing to cross the line, change the timeline. Desperation, poor judgment.

" She's the one who figured me out...all of it."

DYINGSHOOTINGSTAR-

Thad is more patient but some of the same things annoy them too much. 'Sharp they can't stand.' Tapping his pencil at times.