Nerd glasses originally referred to dark and heavy unisex frames, usually made of plastic or acetate, and were generally worn by honest nerds. Nowadays, it refers to those fun, geeky trendy vintage eyeglass frames.
It's more about choosing the shape and material of the eyeglass frame. You need to pick square or rectangular eyeglass frames; thicker, plastic, or acetate material is best. Also, you can never go wrong with choosing black or dark unisex eyeglass frames that look
Geek and nerd glasses are very similar, and they are the kind of thick unisex style glasses frames. However, geek glasses are more acetate material and horn-rimmed shape frames than nerd glasses.