So, yesterday I posted about identifying magic items in AD&D. I didn't even mention that you need Detect Magic to tell things are magical (unless the sword is flaming or lights up a 10' radius . . . probably.)
GURPS is a lot more generous. It has some of the same feel - you need time, effort, and specialized magic to tell what things do. But it is both vastly simplified in comparison and less harsh. And you just get straight up Per+Magery rolls to spot magic items; also there is a spell or two that'll do it for certain, if you want to avoid using your low Per or hit a whole bunch of magic items once. Also the Alchemy skill, plus some time and cost, will let you identify if an item is magic, but not what it does.
These are a little harder to identify. You can't just sip it. Well, you can, and take 10 seconds to analyze it, but any negative effects happen full bore on you right away. If it's a beneficial potion, you need to roll vs. Alchemy skill to identify it. The safe way is 4 hours and a kit.
You just read them and see what's on them, unless it's in code or a language you don't understand.
Other Magic Items
Use might reveal some effects, but the go-to is Analyze Magic. It's 8 to cast and takes 1 hour, and reveals the least powerful enchantment on it and if there is another after that. It's quite useful and reliable in a way that the AD&D spell Identify is neither.
I long ago house-ruled the reverse - it reveals the most powerful first. This made logical sense and meant temporary or minor enchantments didn't obscure powerful magic - the Puissance +1 on an object didn't conceal that it was a Loyal Weapon from an initial scan.
And that's pretty much it - look at or touch the item, read the scroll, use Alchemy on the potion, and cast Analyze Magic on items. Like I said, pretty straightforward. Still, I wanted to go back and check while I was thinking about the harshness of AD&D's admonitions.