One verse

Srimad Bhagavatam 7.11.35

7.11.35

यस्य यल्लक्षणं प्रोक्तं पुंसो वर्णाभिव्यञ्जकम् । यदन्यत्रापि द‍ृश्येत तत्तेनैव विनिर्दिशेत् ॥ ३५ ॥

yasya yal laksanam proktam pumso varnabhivyanjakam yad anyatrapi drsyeta tat tenaiva vinirdiset

yasya
of whom
yat
which
laksanam
symptom
proktam
described (above)
pumsah
of a person
varna-abhivyanjakam
indicating the classification (brahmana, ksatriya, vaisya, sudra, etc.)
yat
if
anyatra
elsewhere
api
also
drsyeta
is seen
tat
that
tena
by that symptom
eva
certainly
vinirdiset
one should designate.

If one shows the symptoms of being a brahmana, ksatriya, vaisya or sudra, as described above, even if he has appeared in a different class, he should be accepted according to those symptoms of classification.