Roos start to develop a large comb; I call it a round gummy. So just be looking out for that. Also a roo has long neck feathers and hens do not; that will be very noticeable later though.
With silkies it is hard to tell.
The comb on yours is not big so far. Maybe you have a hen.
Can you tell me the age? It might be too young to know for certain.
You see the big comb that is growing above his beak? That screams rooster.
Here is another picture of him grown up:
Long neck feathers, taller than my hens, big comb, ect.
One way I found out was his comb really. Some develop it right away others it takes time. Mine was evident at 3months of age (the first picture).
You can't tell gender at this age (although my roo had his big comb by then). I think she was @3 months here?
Here she is at @5-6 months:
Hen as many people here identified her and they were right
No large comb, no long neck feathers, relatively short legs, and no long butt feathers (sorry I don't know the correct technical term for the long butt feathers).