How do you know if somebody has osteochondroma or osteosarcoma? do you have to wait for biopsy results?

Almost never: Osteochondroma and osteosarcoma look very different on x-ray, ct and mri. Osteochondroma can be confidently diagnosed with imaging alone. There is <1% chance of an osteochondroma turning into a chondrosarcoma. In that rare event, it's usually a low grade one that almost never spreads. For osteosarcoma, there is usually strong suspicion based on imaging, but a biopsy is needed to confirm.