Ashtangasamgraha 37.9

37.9

yathāyathaṃ copadiṣṭaiḥ parisrāvaistvagādiṣu śalyamupalakṣayet |

sūkṣmagatiṣu śalyeṣvetānyeva lakṣaṇānyavispaṣṭāni bhavanti ||

And one should detect a foreign body in the skin and the rest by the discharges described respectively.
When foreign bodies have subtle paths, these very signs are indistinct.