Colour of dentin is yellowish while that of enamel is pearl white, secondly if you experience pain(sensitivity) while eating or drinking too much cold like ice-cream and too hot like tea or coffee then yes it dentin is exposed.
Next Steps
There are many treatment protocols to treat exposed dentin some are temporary while some permanent like filling dental cement,using sensitivity toothpastes like sensodyne,vantez etc
Health Tips
Always get a new tooth replaced after any tooth extraction.