Bass bar. Is it integrated? Or glued. Are all the corners blocked fully? or half way? or not at all? Generally fake corner blocks are a bad sign. I like to look for deep ribs and arching because that’s more of a Guarneri style and therefore not one of billions of strad copies (not to write those off I’m just tired of them).
when I cannot view the violin in person:
I also look for figuration on the neck. The neck area can tell you a lot about the quality of varnish used. Does it look like paint? Did they varnish the whole neck? Or are the varnish endings opaque with layers. Purfling is also important. Just make sure it’s there, and don’t be fooled by double purfling, it doesn’t always mean it’s special. The top grain I like to see tight and a feathered sheen. that seems to be a sign of soft and supple wood. The maple back and ribs well figured. (Bonus if they’re one piece) I know this is an unpopular opinion but having a nice chunk of wood when making a violin was not the concern of the factorys in the ‘60s. it also just helps when looking at photos. No they don’t always need to be flamed, but then you really need indicators of workmanship.
and if they are well flamed does the flame change with the angle of the picture? If not then they are dead or fake. Just as a bonus.. try and see if the maple is all from the same block; same flame width and character.
These are only a few things, but they’ve served me well enough. just try to trust your gut on it, and remember to look at the entire violin at times to avoid myopia.