How to tell.....

You could listen to the tube. The 12AX7 will have higher gain.

You could listen to the tube. The 12AX7 will have higher gain.

Are you sure these tubes are interchangeable without consequence? I thought not, but I could be wrong.

1.If you can see the grid thru the holes in the plate structure, the 12au7 will have a more jagged look compared to the smoother look of a 12ax7.
2.Test it in a tube checker as if they are 12ax7's. If they meet spec, they're 12ax7's. If they are way down on the scale, they're 12au7's.

You need a tester.

12au7 and 12ax7 are not interchangable.