Is there any way to determine whether a lump in thigh is lipoma or malignant by physical exam or ultrasound? if it's large (2 in), more suspicious?

Maybe: If mass is soft and movable, it tends to b benign and not malignant but only way to know for sure would b to do a biopsy. Size alone is also not inductive of malignancy. Sometimes, MRI can help but still not definitive.

Most likely benign: One can usually determine with reasonable accuracy between a benign lipoma and malignancy on a routine physical exam. Most should be removed and analyzed by a pathologist to be sure. If there is concern, an ultrasound or MRI can be helpful but total excision is usually the best approach.

Those are helpful: Those are helpful, but definitive answer would come from excision and pathology review.

Need clinical eval: Soft tissue lesions must be carefully evaluated by history and physical examination. Ultrasound , MRI or other noninvasive studies may be indicated if there is uncertainty about the diagnosis, but excision is often the best diagnostic tool and usually definitive treatment.

No and yes: The best way to know whether a lump is malignant is with a biopsy. If it is a lipoma, physicial exam has classic characteristics, and it feels like a lipoma. If it is a cyst, an u/s is virtually diagnostic, and an MRI can diagnose a lipoma easily. If it is a lipoma, then there is a very, high likelihood that it is benign. If large, or any complexity on imaging, can rarely be malignant.