Observant characters can be difficult to play without metagaming, but you must keep in mind what the other characters are doing. How long did your character actually get a glance at them? What kind of surroundings are there - is there any interference with their sight? How well does your character actually know what they're looking at? 

In the case of the two weapons she has mentioned, and automatically deducted precisely what they were, it was unfeasable. Argent cleaned his sword, yes, but the only mention of his gun form was a minute, quick check of the barrels before he put them away. Even if your character glanced over at him at the time, The most she would see was either the sword itself, the guns themselves, or it starting to shift between froms simply due to the sheer amount of people that would have been wandering around between your characters, blocking her view. In the case of Cereza's weapon, I tried explaining that it was in it's compact form - literally a rectangular box with three blades folded against the side. Cereza was not within Tierra's eyesight when she had the weapon unfolded, and there are plenty of weapons that collapse down into boxes with blades -  Crescent Rose is the main one I can think of and a wide variety of gun-swords and -axes, as well as the polearms. For Tierra's mind to automatically leap to polearm when the weapon was hidden behind Cereza herself and she was trying to make sure Cereza wasn't hurt, it was simply too contrived.

With character interactions, unless your character is a highly suspicious and paranoid about the underworld kind of character, or a snob, they would automatically think normal jobs for anyone they meet. A thin, almost sickly girl would be seen as someone who might run messenges rather than automatically being a thief, as Tierra's personality and background don't give her the mindset that would make that connection.

So, you need to keep an eye on what other characters actually do and how long they do it for, where they are in relation to your own character, what the surroundings are like and how they could interfere with anything she may notice, and how well does your character actually know what she is looking at. However, if a character states something clearly in their interaction with her or an event is made clear in a GM post that affects Tierra in some way, she can use that information as any person would, drawing her own conclusions or questioning the others about it more.

We wish to assure you that this isn't an admonishment, but simply our attempt to make you aware of ways to cut out metagaming while still playing an observant character. Penalties are a possibility, however we would prefer not to do so and give you this chance to improve on your roleplaying skills and character.