It's been a while since I played Diablo2 (before the latest patch), so I'm not 100% sure if this is still accurate.
These were the TPPK builds that I encountered while I was playing Diablo 2 (more or less in chronological order):
- Guided Arrow Amazon (is no longer a common PK build)
- Hydra Sorc (also not popular anymore, AFAIK hydras disappear when entering TP)
- Trap Assassin (traps disappear when entering TP)
- Tornado Druid (not very popular anymore)
- Meteor Sorc (rarely seen)
- Blessed Hammer Paladin (also quite rare)
- Blizzard Sorc
- Ice Blast Sorc
- Bone Spirit Necro
Some PKs really seem to enjoy killing low level characters by offering their help and then kill all the noobs at a boss-fight. When some high level Player (nobody from your party knows) offers help, you should really be wary.
Stay alert when going to Waypoints, especially if you're going to ones that were given by another Player (potential PK) or ones that the PK could guess (you're going to enter Worldstone Keep in a Baal run aren't you?).
Areas like the Throne of Destruction are popular for PKs. All the players are gathering at one place, being mostly immobile. These are easy to kill. You should pay attention to other players positioning themselves behind you. Especially look for Cold-Sorcs with the obvious fast-cast gear (Skin of the Vipermagi is easy to spot). Also Necromancers using Bone/Spirit or Teeth should ring some alarm-bells.
The thing about hardcore is, that you cannot enjoy it without worrying. You really need to be careful in public games. Have a good look at the other players, try to guess their equipment. Check the minimap and party-screen often to see where players are and where they go. Don't stay immobile..
After some time I became really good at spotting PKs. After all, there aren't soo many different builds and tactics. My characters mostly died due to lag (curse you battle.net) or some other stupid mistake.
Update: I remembered some more stuff to consider, so I'm adding this to this answer.
Never ever engage in a fight if some other player hostiles you, even when he is at a much lower level (unless you really want to get rid of your character). I did some low-level PKing when I was actively playing and killed level 20-30 characters with my level 18 easily. A character built for PvM won't stand a chance vs. a PvP optimized build.
At lower levels (9-29), PKs usually are: Paladin (Might or Concentration aura), Assassin, Throw Barbarian, Frenzy Barbarian. Other characters are rarely seen as PK at these low levels.
Always be wary when a high-level sorceress enters the game and idles or leaves. She could enchant lower level characters which then have a huge damage output. Something you don't want to mess with.
Usually it's best to leave the game when another player hostiles you. You should be able to Save & Quit really fast. Another good option (the one I prefer) is to take a portal to town immediately. That's why I always kept Town-Portal scrolls in one of my belt-slots. It's one of the fastest ways to open up a portal and also frees up space in your inventory (put some nice charms there).