I have caught a few king salmon now, but I'm not sure how to tell a buck from a hen. Do you guys know any ways I can distinguish a buck from a hen? Thanks.

The hens usually have a shorter mouth and bigger belly.  Bucks usually have a longer mouth and are tighter body wise. Hens if they are ready will be dropping their eggs ( you can give their belly a gentle squeeze) and bucks if you do that will spill their milk. 

 

Buck:

 

Hen:

Bucks develop a kype when spawning. Hooked jaw like this.

 

If it shoots out milt then it's not a hen.

1 hour ago, UPSmallie said:

If it shoots out milt then it's not a hen.

Lol, that's also a tell-tale sign.  Same would go for eggs squirting out.

Here's a couple of some rainbows.

 

Buck:

 

Hen: