traits of a Roman legionary

[Tarbicus]

If a recently retired legionary walked into Flavia's atrium in an ordinary tunic
and sandals, how could she deduce -- Sherlock-Holmes-wise -- that he had served
in the Roman army?

Look to the Heculaneum soldier: Bow legged; broken and missing teeth (unlike the others found on the beach with him).

A common term for a soldier was 'caligatus', so he'd probably wear caligae. Perhaps (let's face it, his pugio) an ornate dagger.

Depending on the period, he wore his tunica hitched up high?

According to a book I'm currently reading, dissatisfied with being a farmer. Thinking of re-enlisting? No matter what, though; lots of stories to tell, compared to the peasants around him who he might have originally been like.

Maybe.