How can you tell if a pair of tubes are matched?

I believe the only way to be sure is with a tube tester.

The values of the grid current matched for the fixed anode voltage.
Another words if you're setting your trim pots on the approximately same position for all tubes than they're matched.
The main point that the offset should be same especially for the power tubes that may work peak to peak. With unmatched tubes you may experience bad performance on the high volumes.

Check eharmony.com for compatibility.