It was easier to spot back in the day as the ink would be smudged, as your hand went over the recently applied ink; rather than away from it.

I only know by 'ticks'.

Yep, the slant on the letters, and 'backward' ticks give it away. I'm left handed also and yes, with poor pens you can smudge but not so much anymore.

Off the top of my head as someone who is left-handed, ink/lead smears would be a giveaway, though TBH normal pen/paper doesn't tend to smear much. Some lefties write in backhand, in which the letters tilt to the left rather than to the right. However, plenty of left-handed people will write by twisting their wrists so that (assuming the paper is perpendicular to the desk) their knuckles point to the top of the page. But then others will turn the paper a bit counterclockwise instead of bending their wrist. On a spiral notebook, a lefty is obviously more like to write so that the spiral isn't digging into their wrist. In terms of things like quick notes, I would say that a left-handed person is far more likely to scrawl something in the top right corner than the top left because they can rest their hand on the paper rather than the writing surface.

As far as the person themselves go, if you look closely at the outer edge of my left ring finger, you could probably tell that I write with my left hand because the skin is a bit harder. I would think right-handed people would have the same thing. If they write with pencil, they might have a silvery smudge between their bottom knuckle and their wrist.

if there are any other lefties that disagree, I would be happy to hear because I'm using myself as a reference.