How to tell if tubes are matched?

First the tubes are packed in their own boxes. Each box will have the respective tube socket number written on each of them. Or at least each tested match pair. The owner’s manual will have a block diagram showing the tube layout. Your amp may even have the socket number stamped be side each of the sockets. This is for a new amp. If you are buying a used amp, hopefully the seller kept the original tube boxes, and as he pulled the tubes out put them back into their respective box. Or at the very least identified them in someway. If they are not identified...contact the seller.

Is this amp new or used? If used there is more you need to know.
Jim

Relax. This is not a new problem. Normally only output (large) tubes are matched. If input (small) tubes are not marked "left" and "right" etc., it won't matter. The output tubes will be marked as pairs in some fashion. Before you rip the boxes open, look for rubber bands holding pairs (or quads) together. Alternatively, the boxes themselves will be marked. I store my tube boxes in the exact same order that the tubes are installed in my amps, so if I try another set ,the tubes always go back in the original boxes. Stay cool, and if when your amp arrives you are not certain, post again with specifics, and somebody will walk you through it.
Do make certain that you plug the small input tubes in the correct sockets, if you get them backwards (i.e. put the 12ax7 in the socket intended for the 12at7, the darn thing might not work. Read the instructions concerning output tube bias carefully before you proceed. Tubes are tough and will withstand abuse, but improper bias will kill them off in a hurry.
Finally, tubes have been around for almost a century. If they were difficult and delicate flowers, they would have been abandoned long ago.

If the output tubes are way off, you might have problem adjusting their bias. But in general, there is no way to tell if the two tubes are matched without running them through test equipment.