No, you cannot tell from the external characteristics of a cannabis seed whether it is feminized or not. Feminized cannabis seeds have been genetically engineered or treated to ensure that they produce almost exclusively female plants, but this does not affect the appearance of the seed itself.
The only way to be sure that a cannabis seed is feminized is to purchase it from a reputable source that guarantees that the seeds are feminized. When planting seeds, it is only after germination and the plant grows into the vegetative stage that you may start to see sexual characteristics, with female plants developing white hairs or pistils at the nodes.
If you have seeds and want to know if they are feminized, you will have to rely on the supplier's information or grow the seeds to determine the gender. It is important to note that even with feminized seeds there is a very small chance that a male plant can develop, although this is rare.