Usually easy way to see who has autoblock is that they are really bad at attacking. If they are extremely good at blocking, they should also be good at attacking. More experienced players can spot this kind of actions quite easily. Also hold attacks works great against autoblockers, never had any problems with autoblockers. In sieges there are mostly group fights, just see where your teammate is hitting the enemy and hit from the other side. This way even 2 newbies can easily kill pro/hacker.
What do you mean with the holding attack? They are kind of stuck with the block direction when you are holding? I think the "best" auto blocker will block at the last millisecond before they get hit. So they are kind of all the time attacking and then when your swing is going trough, they will block and the block lasts for only a few milliseconds and then they are again attacking. It's kind of unnatural. At least for me.
That tg-arena is so much more cancer that the siege. There everyone is alone and I think for there it has a much smaller threshold for them to start using some automatic tools. The problem there is that most maps are too small for the 60 players that there often is online. You just spawn somewhere in the middle of someone's else fights and you get killed before you even realize that you even were alive for while.
That kind of maps are for example arena (even tho many players love it) and that hecking dark banquet room with lots of tables (can't remember the map name now.) So when you got like multiple times in row backstabbed, then some of us will get so salty that they start finding cheats. I've seen in some forums people asking cheats because of that specific reason. Other reasons are, that they just wanna "punish" some players they think are annoying. The TG DM community is just like that, full of annoyed people who hate each other.
About the fire arrows. Yeah, I use them a lot. If you hit to the upper part of the person, or to the side, you more easily fire the dude up than shooting to the middle of the shield. Also for example in TG-arena, there are fire arrows in the dungeon, so if you hang out close to it, you can get more arrows easily from there. From orro mine you can get pistols from the tower.