Yeah, that's true. For example to figure out my Typhlosion's Hidden Power I went up against a Zubat, Hariyama, Oddish and Numel. First it was the Numel which it did normal damage on. Then I went and faught Hariyama. Now keep in mind this is in Emerald version. So I faught it, and it also did normal damage. But at that point I also used Flame Wheel, and realized, that Flame Wheel did just about as much damage as Hidden Power. So... eventually Hariyama whirlwinded me out. Then I went against Zubat, it was Super-Effective. Zubat is weak against: Ice, Electric, Rock and Psychic. Can't be Electric because it affected Numel (Fire/Ground). Can't be Rock because it did normal damage on Hariyama (Fighting, which is resistant against rock) and it can't be psychic because it was normal against Hariyama. So it was Ice, which was then confirmed by attacking an Oddish. And comparing it with Flame Wheel, it actually did a bit more damage against that Hariyama, so I was certain that it was pretty strong, higher than 60 Base Power.
An easier way to find out your type though, is by going against Kecleon. As Kecleon's color change will immediately change it's type to match your Hidden Power's type. I later checked on Serebii, and that was the case. It was a 65 Base Power Hidden Power Ice.