How do u tell if a sin has slashing or piercing daggers?

So been monking for a while with my diff shield sets, but I just realised that I have always assumed that daggers were slashing. Well I found out that they can be both.

Is there any way you can tell if the sin that is attacking you is using slashing or piercing daggers so u can use the right shield?

If you can catch the animation for their regular attack, it'll be immediately obvious whether they're slashing you or poking you. Unfortunately, the attack skills tend to use their own animation regardless of dagger type, so if the assassin never auto-attacks you'll just have to guess.
Alternatively, you can try to memorize dagger skins, as certain skins will always slash, while others poke.

have someone scrim you and remember the damage u take when u have the correct shield on... the damage will either be higher or lower if u have the correct shield on, if the damage is higher, then u know to switch.

That's more skins than I think you realise.

Bonus damage from most skills is armour ignoring. Base damage from daggers' auto attacks is negligible, you don't have to worry about it. Unless they have a conjure on, then switch to the shield for the element.

On the plus side there are tons of sins running around with claws of the broodmother, sai,stilletto's, golden talons etc. ( the cheapest ones skin wise).

How can you tell if a sin is using a conjure? Do you have to just get lucky and catch them casting it before-hand?

Either that, or you see A/E, which almost automatically tells you:

"Hi there guys, I have a conjure!".

Speaking for sins, that is.

generally, most sins use piercing daggers.

there are a few types which are slashing such as ungues of the forgotten, the shashing dags usually have a fairly recognisable skin, so just lear which ones do slashing dmg.