Normally it's done by the shape of the top cross bar at the top of the sliding carriage. The carriage is the part that slides up and down the tower, holding the gear works, and handles.
If the bar is a long arc then it is a Snell, if it had a bend in it like the top of a heart or like an m then it is a James Swan & Company out of CT. If it is an all metal tower it could be an Ajax which has a hollow chuck and a hole up through the top of the tower's cross bar for extra long bit shanks. Or if it has a metal tower and two chucks it is a Boss double eagle. Or if it has a metal tower and the carriage slides up and down two round metal rods and two long rods coming from the tower top down to the seat it is a Millers Falls.
That's about it for the most common brands.....
