First, the facts. 3TTT makes some stems in 26.0, and generally it says on the stem down below the insertion line.
Sadly, sometimes it doesn't. Also in 26.0, some Modolo, ITM, and Cinelli. Probably others, I'm just not up on them.

Cinelli 26.0 stems will always, in my experience, say so on the stem.
However, most of them are 26.4, so be careful.

Putting a 26.0 bar in a 25.4 clamp is possible. You need to wedge the clamp open far enough to get it on, make sure it's where you want it when you remove the wedge, because no matter how you move it unwedged, you're going to scratch the bar. There is no guarantee it will work, and no guarantee it will not break the stem clamp. I'd not do it, but it's your choice. I'm sure others have done it.

26.4 into a 26.0 clamp is a bit easier, same process, still not recommended.
Based on the popularity of Cinelli bars, probably done a lot.

Many Nittos have the bar diameter on the end of the drops, but not all.

The whole problem is the older bars and stems were 25.4, except for the occasional 26.0 or 26.4.
When quills ruled, there was plenty of selection.

Now, with 31.8 and 26.0 being much more prevalent with the removable stems, there are more 26.0 bars made.
We, in C&V, are in the middle of that mess.