Which reminds me of this:
Russell Peters on the racism in ASL. NSFW language, and pretty awful if true.
Except The West Wing. Her character had an interpreter, and some people tried to learn a few signs to communicate with her, but I don’t remember anyone using the “oh, I have a deaf relative so I’m fluent” gambit. I’m sure if I’m misremembering, someone will correct me.
It always bothered me a bit that William Hurt, in Children of a Lesser God, translated his dialogue and Marlee Matlin’s (obviously for the audience) during their “conversations.” Subtitles would have been more realistic, wouldn’t it? Or have I missed something? Still, an excellent movie.
Perhaps. But I suppose audiences tend to want to be able to look away briefly, to hear instead of constantly looking at the screen. As Lily Tomlin so accurately noted “they call it show business, not show art”.
…who knew sign language. 
Sam picked up a working knowledge of sign language in QUANTUM LEAP, right? Came up in multiple episodes, if I remember correctly.
We have an old photo of my dad doing this very thing. To my knowledge, I have no relatives who know ASL.
Yeah, I have a deaf relative, but I’m very far from fluent. It’s much easier to sign than it is to understand signs. If they’re going at deaf speed I can barely recognize any signs, it’s all a blur.
On the other hand, my brother and sister in law, who are the parents of my deaf nephew, really are fluent. She works as an interpreter and he works as a special education teacher for deaf kids.
They also briefly show up as children in Season 2, and are seen signing with each other, so they go back a long way
Camryn Manheim guest starred in a 1993 episode of L&O, where she represented a deaf suspect.
Renee Zellweger’s character in Jerry Maguire knew sign language.
Going way back, 87th Precinct was a 1961-62 TV series based on Ed McBain’s novels. I never saw it, but I’ve read the novels, and Detective Steve Carella (Robert Lansing) was married to a deaf woman, Teddy, (Gena Rowlands) and in the book they signed to each other. (He learned it after meeting her.) Teddy was deaf in the series, too.
I was going to say “The bad guys in Fargo - both the TV show and the movie” but I didn’t know that the TV actor himself is deaf. Thank you!
Susie in “Curb Your Enthusiasm” shows knowledge of ASL in at least one episode.
It was only brief scene in ER, but Dr “Rocket” Romano signed to Dr Benton’s deaf son. It was never explained how Romano knew sign, but the character had a number of surprising little talents/skills.
Of course the potential victim they were trying to save DID happen to be completely fluent and had several cutesy conversations (in sign language) with the deaf hunter talking about which of the Winchesters they would rather sexually harass.
Silas Botwin in Weeds. I can’t believe I got in this late with him. It’s a great show.
Oh! In the Sherlock special, Victorian!Holmes speaks fluent BSL, and Victorian!Watson tries but is amusingly bad at it.
Watson: (in BSL) “I’m glad you liked my potato,” to someone who praised “The Blue Carbuncle.”
Deadpool knows ASL and uses it to communicate with Clint Barton (Hawkeye), he also lifts up his mask so that Clint can read his lips.
