And if so, how would you know to go on a night when they’re cooking?
No, in most cases. They tend to consult on the menu, but you won’t find them on the line, cooking. There are exceptions, of course, and some of the newer celebrities (like the ones who emerge from Top Chef) definitely cook, but if you’re talking Bobby Flay or the like, chances are they are not in their restaurants cooking very much, if at all.
Even if they are at the restaurant, and in the kitchen, they are more likely to be at the pass (although not necessarily running the pass), than actually standing in front of the stove so to speak.
I presumed as much. 
One can indulge in wishful thinking though.
When I went to Chicago and ate at Frontera Grill for a late lunch, Rick Bayless was in the kitchen. He came out and ate his own lunch at the counter near the end of service.
Tom Colicchio sed to have “Tuesdays with Tom” at Craft, which advertised that he would be in the kitchen. Don’t know if he still does this.
Even when famous chefs are in the kitchen, they are more likely to be expediting rather than on the line.
Best time to ensure the chef is in the house is when they open a new restaurant.
I actually asked about this when my wife and I dined at Emeril’s in New Orleans. We were told that he shows up about a once month, and does work the line when he’s there. This was six years ago, though.
Thanks!
Top Chef’s Stephanie Izard was shown cooking on the line at her Chicago restaurant the Girl and the Goat. Anthony Bourdain visited there in a recent No Reservations episode and was very complimentary. I was surprised at how big the Girl and Goat is. Looked like she could seat at least a hundred people there.
Even without the chef being a celebrity, he might still not directly handle the food himself. The title “chef” is cognate to “chief”, and just means he’s the guy in charge in the kitchen. You could just have a chef barking orders to everyone else who’s doing the actual food preparation.
She’s nice as hell too. A few years ago my daughter and I were walking by TG&TG one Saturday morning (there are a lot of specialty/wholesale markets in the neighborhood) and as we passed by we looked in the window and saw her talking with the staff so we peeked in. She came out and said Hi, and we mumbled something about being fans and left with a smile.
I’ve read somewhere that Ming Tsai is still quite involved with his restaurant in suburban Boston. I’ve never been, so no sightings to report.
Pretty much what I was going to say; a chef has the same relationship with the preparation of the food as an architect has with the construction of a building. There are some chefs who work on the line, but there’s also nothing except the union, I guess) stopping an architect from putting on a hardhat and picking up a rivet gun.