Usually a lag switch is involved when the game starts being all laggy. This might sound obvious, but a good way to know a lag switch, not the host's connection, is used when you start teleporting everywhere.
If you stop teleporting and are still in the game, someone has a lag switch. If not, the game will end.
Precer posted... Or that could just be a bad host. I can't host because I don't have a good connection. When I do host I run around killing people and I don't figure out others are lagging until I get booted... --- the last time I played yo falcon I found out I had a little brother on the way. ~Narukuboy http://www.youtube.com/watch?v=fomKPsbGCHM
You basically can't tell 100% unless you saw him doing it in person. A terrible connection can look like exactly the same thing.
best way to tell is to push select as the game is loading for you. If someone has a five-bar while everyone else has a one-bar, keep an eye on him.
This is just how you determine who the host is. Doesn't mean anything unless it stays that way during the game.
If you're running around and the game starts lagging out, followed by instant death, chances are you're getting lag switched. Especially if it happens repeatedly. This is just from my experience though, I'm not an expert on the matter.
I lag for real all the time and it's a little different when someone uses a lag switch so I can tell right away usually. And since I'm so used to playing with lag, I usually still win.
Normal lag: You'll usually run for a few seconds then you'll snap back to where you were. If you spin in circles, it'll go for a second, then your camera will get stuck for a split second and keep going.
Lag switch: You'll usually run in place for a few seconds then snap ahead about 30 feet. The spinning is pretty much the same as normal lag.
I toggle thermal because I prefer my targets to be black. -Lykos13th