Identifying the Conjugation

To identify the conjugation, you use the second principal part, called the present infinitive, the infinitive endings tell you how each verb is grouped, and how each verb in conjugated. Follow the conjugations rules very carefully

Infinitive Endings

1st Conjugation- amo, amare, amavi, amatus - to love
2nd Conjugation- sedeo, sedere, sedi, sessurus - to sit
3rd Conjugation- curro, currere, cucurri, cursurus - to run
4th Conjugation- audio, audire, audi, auditus - to hear

The first and fourth conjugation are very easy to spot from the Principal Parts, but the second and third have the same infinitive ending [-ere, although pronounced differently.]
Simple rule for knowing which conjugation the -ere infintive verb belong to:

-eo for the first Princpal part is 2nd,
not -eo for first principal part is 3rd.