In अजगरपर्व within वनपर्व of महाभारत, there is an interesting event where one of the ancestors of the Pandavas, King Nahusha, takes the birth as a python after being given a शाप by ऋषि अगस्त्य. And when भीम walks around in the forest, the python coils around भीम who is unable to escape. As it was about to eat भीम, युधिष्ठिर approaches the python asking if there was any way in which the python may free भीम. The python replies that if युधिष्ठिर replies to questions raised by the python, भीम will be freed. An extract of this exchange is an interesting question put up by the python – how does one know who is a ब्राह्मण and who is a शूद्र. Read on…

सर्प उवाच

ब्राह्मणः को भवेद् राजन् वेद्य किं च युधिष्ठिर। ब्रवीह्यतिमतिं त्वां हि वाक्यैरनुमिमीमहे।।

O राजन्! Whom can we call a ब्राह्मण. O युधिष्ठिर! What is it that ought to be known? From what you have said, I deem you to be endowed with very high intelligence.

युधिष्ठिर उवाच

सत्यं दानं क्षमा शीलमानृशंस्यं तपोघृणा। दृश्यन्ते यत्र नागेन्द्र स ब्राह्मण‌ इति स्मृतः॥

O नागेन्द्र! One is a ब्राह्मण‌ in whom are found सत्य, दान, क्षमा,‌ शील (good conduct), अनृशंस्यं‌ (benevolence), तप & अघृणा (lack of hatred or merciful). 

वेद्यं सर्प परं ब्रह्म निर्दुःखमसुखं च यत्। यत्र गत्वा न शोचन्ति भवतः किम् विवक्षितम्।।

O सर्प! That which ought to be known is परं ब्रह्म which is a state devoid of र्दुःख & सुख and reaching which creatures are past all misery.

सर्प उवाच

चातुर्वर्ण्यं प्रमाणं च सत्यं च ब्रह्म चैव हि। शूद्रेष्वपि च सत्यं च दानमक्रोध एव च। आनृशंस्यमहिंसा च‌ घृणा चैव युधिष्ठिर।।

O युधिष्ठिर! Even in the शूद्र are found सत्य, दान, अक्रोध (no anger), अनृशंस्यं‌, अहिंसा, अघृणा and knowledge of the वेद which promotes the welfare of all four वर्ण, which is true and guide in all matters of धर्म.

वेद्यं यच्चात्र निर्दुःखमसुखं च नराधिप। ताभ्यां हीनं पदं चान्यन्न तदस्तीति लक्षये।।

And O नराधिप! That which is to be known is asserted by you as a state of no दुःख & सुख; but I do not find such a thing in which these feelings are absent.

युधिष्ठिर उवाच

शूद्रे तु यद् भवेल्लक्ष्म द्विजे तच्च न विद्यते। न वै शूद्रो भवेच्छूद्रो ब्राह्मणो न च ब्राह्मणः॥

The शूद्र in whom these लक्षण are present is not a शूद्र but something higher and a ब्राह्मण in whom these लक्षण are absent is not a ब्राह्मण

यत्रैतल्लक्ष्यते सर्प वृतं स ब्राह्मणः स्मृतः। यत्रैतन्न भवेत् सर्प तं‌ शूद्रमिति निर्दिशेत् ॥

And O सर्प! It is asserted that the one who is distinguished by these लक्षण is a ब्राह्मण and who does not possess them is a शूद्र

यत् पुनर्भवता प्रोक्तं न वेद्यं विद्यतीति च। ताभ्यां हीनमतोsन्यत्र पदमस्तीति चेदपि।।

एवमेतन्मतं सर्प ताभ्यां हीनं न‌ विद्यते। यथा शीतोष्णयोर्मध्ये भवेन्नोष्ण न शीतता।।

एवं वै सुखदुःखीभ्यां हीनमस्ति पदं क्वचित्। एषा मम मतिः सर्प यथा वा मन्यते भवान्।।

Again, as regards your remark that the aspect to be known does not exist for, nothing that is devoid of these feelings can have any existence. It seems (indeed at first sight) that existence is not possible  without these. But as cold is characterized by absnece of heat and heat as the absence of cold. So cannot there exist an aspect or state characterized by the absence of both these feelings? O सर्प! That is my opinion, what do you say?

सर्प उवाच

यदि ते‌ वृत्ततो राजन् ब्राह्मणः प्रसमीक्षितः। वृथा जातिस्तदाssयुष्मान् कृतिर्यावन्न विद्यते।।

If, O राजन्! a ब्राह्मण is recognized by certain traits, then the distinction between जाति is to no purpose as long as he does not possess these qualities 

युधिष्ठिर उवाच

जातिरत्र महासर्प मनुष्यत्वे महामते। संकरात् सर्ववर्णानाम् दुष्परीक्ष्येति मे मतिः।।

O महासर्प! I think here in this world, it is very difficult to ascertain one’s जाति on account of continuous intercourse between people of all‌ वर्ण

सर्वे सर्वास्वपत्यानि जनयन्ति सदा नराः। वाङ्मैथुनमथो जन्म‌ मरणं च समं नृणाम्।।

Men of all वर्ण are without restriction constantly be getting children on women od all वर्ण. And speech, cohabitation, birth and death of all people are similar in all respects. 

इदमार्षं प्रमाणं च‌ ये यजामह इत्यपि। तस्माच्छीलं प्रधानेष्टं विदुर्ये तत्वदर्शिनः।।

The proof of this, ie the difficulty of ascertaining one’s वर्ण is found in such expressions made use of by the ऋषिs, as “whatever वर्ण one may belong to, we celebrate the यज्ञ”. It is on this account that the wise have asserted that the character is the chief and needful thing.

प्राङ् नाभिवर्धनात् पुंसो जातकर्म विधीयते। तत्रास्य माता सावित्री पिता त्वार्चाय उच्यते।।

The natal ceremony of the male child is performed even before the severence of the umbilical chord. On that occassion, his mother is designated सावित्री and पिता as the आचार्य

तावच्छूद्रसमो ह्येष यावद् वेदे न जायते। तस्मिन्नेवं मतिर्द्वैधे मनुः स्वायम्भुवोsब्रवीत्।।

कृतकृत्याः पुनर्वर्णा यदि वृत्तं न विद्यते। संकरस्त्वत्र नागेन्द्र बलवान् प्रसमीक्षित:।।

Before initiation into the वेद, every man is regarded as a शूद्र. There being a difference of opinion on the point, स्वायम्भु मनु laid down that if having gone through purificatory rites (as laid down in the वेद), the first three वर्ण do not regulate their conduct, in that case, tyr mixed castes should be considered as superior to them. 

यत्रेदानीं महासर्प संस्कृतं वृत्तमिष्यते। तं ब्राह्मणमहं पूर्वमुक्तवान् भुजगोत्तम।।

O महासर्प! I have therefore designated him as a ब्राह्मण who observes the principles of good behaviour. 

Source:

Translation of Mahabharata by MN Dutt