How am I supposed to able to tell that:
Monere contains a long e
Regere contains a short e ?
Experience. Also, in the case of verse, you can work out whether a vowel is long or short by scasion.
If you are wondering how to indentify conjugations based on the infinitive endings, you can’t. You may be able to tell 1st conjugation by the -are infinitive, and 4th conjugation by the -ire infinitive, but unless macrons are marked, then you cannot do the same for 3rd and 4th conjugations.
Instead, you must look at both the first principal part and the second:
if the first principal part ends in -eo and the second in -ere, then the verb is second conjugation. e.g. caveo, cavere; albeo, albere.
if the first principal part ends in -o and the second in -ere, then the verb is second conjugation. e.g. capio, capere; gero, gerere.
the deciding factor then is whether there is an e before the o in the first person present.