I came across this in chainsaw-man vol 12 ~ page 90.
I think it’s たっ but I’m not sure. If so, I am not certain what it represents.
In the English translation (i found online) it’s “tmp”.
just a sound effect, whatever the character is doing made a たっ sound. There seems to be lockers in the background, so maybe the sound of one opening or closing or the lock or something.
What I did was just mess around with a load of fonts and enjoyed comparing them. Other than that, it’s just reading more things that aren’t just plain text.
She doesn’t open the locker until the next panel, so based on the hair swish I think it’s supposed to represent a heavy footstep, like coming to a sudden stop.
If you read a lot of manga you’ll get plenty of practice. For me the issue isn’t usually to recognize the characters but rather to figure out what kind of sound or concept they’re supposed to convey.
There are some online resources for that: e.g. Japanese-to-English SFX Sound Effects Translations - a @ The JADED Network seems to have a pretty wide selection with images of use (though the site hasn’t been updated in a decade). But I mostly just guess or assume based on context and the onomatopoeia words I know, and ignore the sfx I don’t really understand – after all they’re basically always skippable without missing anything important.