I was wondering if thyroidectomy the only way to determine if a papillary lesion / follicular neoplasm is benign or malignant?

No/Yes. Papillary cancers can usually be diagnosed by fine needle aspiration biopsy (fnab) preoperatively. In contrast, the distinction between a benign and malignant follicular neoplasm can only be determined by removing the thyroid lobe that contains the nodule. In these situations, the pathologist is looking for cellular invasion into blood vessels or the capsule of thyroid: this cannot be seen by fnab.
Thyroid FNA. There are several different results for fna biopsy. Benign 5%risk of cancer, follicular lesion of undetermined significance (flus)10% risk of cancer, follicular neoplasm 30% risk of cancer, suspicious for papillary cancer 70% risk of cancer, papillary cancer > 90% risk of cancer, generally a diagnosis of follicular neoplasm requires lobectomy. Flus should be repeated in 3 m with veracyte test.