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I have two old classical guitars with no way of knowing who made them, or finding a specification sheet, or something similar. I normally play with high or ultra high tension strings, but I don't know if those guitars are designed for high tensions, so they've had normal tension strings since I got them.
Which got me thinking, is there a way to know if a classical guitar is designed for higher tensions? Maybe through their materials, shape, form, or design particularities?
Unless the guitars are individually built guitars and not "shop" produced, then they likely follow standard, traditional construction. I have personally never run across building instructions that take into account different string tensions in regards to the face and bracing, although I am not a classical guitar maker, and things may have changed or be different from when I started studying Lutherie.
If the guitars follow traditional bracing and face thickness, then what is sold as "high tension" strings should be in the range of tension that will work on the guitar. The tension range between the High and Low varieties isn't very large, and what is considered High and Low varies between manufacturers, so one's "Medium" is in the range of an other's "High".
As stated in Francis Phillips' answer, it is worth a trip to a Luthier if you have doubts about these particular instruments. What I would look for if I was inspecting is the internal bracing on the face. If the bracing looked especially thinner than standard or the instrument construction looked like it pre-dates the use of Nylon strings, I would recommend lower tensions. If they are reasonably modern and the bracing looks normal, then you should be fine with modern string tension ranges.
Best approach is the conservative one. Find a luthier and have them evaluate the instrument. Using high-tension strings on older instruments can possibly cause irreparable damage.
