Here is how to tell if your frame is Chromoly or Hi-ten steel:
1- Price. Usually Hi-ten is much cheaper than Chromoly steel.
2- Weight. Because Chromoly is stronger it uses less material than Hi-ten
3- Tapping. Just by looking at the bicycle it is hard to tell what material it is. If you have a pencil tap the frame. If it sounds like "Ding" instead of "Donk" it is probably Chromoly.
For a video please check: