Can you tell what a tag holds by looking at it?
No. A tag has no display and no marking that reflects its content, so two visually identical stickers can hold completely different things. The only way to know is to read it, which is why labelling your own tags and reading unfamiliar ones are both worth the habit.
The content is invisible
A tag is a chip and a coil. Nothing about its appearance is affected by what is written on it, and nothing ever changes when it is rewritten.
The printing on a tag, where there is any, was applied by whoever made or bought it and has no connection to the content.
So a drawer of identical white discs is a drawer of unknowns, and the only way to sort them is to read every one.
What follows from that
You cannot inspect a tag before tapping it. There is no equivalent of hovering over a link, and a tag on a wall tells you nothing about where it leads.
A substituted tag is undetectable by eye. Somebody replacing yours with theirs leaves a scene that looks identical.
A tag you wrote last year holds whatever you wrote, and you will not remember. This is the failure that actually costs people time.
And a deployment without a record is a deployment nobody can audit, since the tags themselves will not tell you what they were for.
Labelling your own
Print or write on the outside what the tag does. A word is enough and it is the difference between a set that gets used and a set that gets abandoned.
Keep a record of which tag is where and what is on it, captured while writing them rather than reconstructed afterwards.
For a deployment, record the identifier as well, since that is what lets you tell a substitution from the original.
None of this is exciting and all of it is what separates a deployment that survives two years from one that becomes a mystery.
Reading somebody else's
Use an application that displays the content rather than acting on it, which turns an unfamiliar tag from something that happens to you into something you inspect.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
Look at the address, not just the fact that there is one. A plausible looking domain is the whole of most deceptive tags.
This single habit covers nearly every concern in this section of the site, which is why it appears in so many of these pages.
Frequently asked questions
Can I tell a tag's content from its appearance?
No. Nothing about a tag changes when it is written, and printing on it was applied by whoever supplied it.
How do I keep track of my own tags?
Label them physically and keep a record while writing them. Reconstructing it afterwards means reading every one.
Would I notice if somebody swapped a tag?
Not by looking. Recording the identifier when you deploy is what makes a substitution detectable.
How do I check an unfamiliar tag?
Read it with an application that shows the content instead of acting on it, then judge the address.