The first thing that will help you readily identify the aorist is to master all six principal parts for each Greek verb you learn. As William pointed out the "sigmatic" aorist has a sigma added to the stem, which sometimes remains visible as a sigma (as in the verb "luo" ---> "elusa") and sometimes combines with the consonant in the verb stem to produce one of the double consonants ( as in "pempo" ---> "epempsa", where -ps- represents the Greek letter psi) but be careful, this could be a future indicative as well, or an aorist subjunctive, and maybe even any number of other things that I can't think about right now, and probably don't matter. Again the first best thing you can do is to master the principal parts of the verbs you learn, and drill yourself over paradigms. Pick a new verb each morning and write out its entire conjugation, one a day. The patterns will slowly start to emerge, while you are training yourself in this way and also developing stronger writing muscles.
Look for these elements as you continue your reading:<br /><br />(1) an initial epsilon. the past indicative augment. Remember that the aorist often indicates past time, and that indicative
verbs take a past indicative
augment. This does not happen, however in the Subjunctive, Optative, Infinitive, Imperative, or Participal, which is why this epsilon augment is called the past "Indicative" augment. Remember that.
<br /><br />(2) in the sigmatic aorists, the tense vowel of the aorist is an alpha, not an omicron or epsilon as in the present and future indicative tenses. But as "klwps" has already alluded to, this is only the case with the aorist active and middle. The passive will often have an aspirated stem consonant. (in the same verbs above "luw" ---> "elu8hv" and "pempw" ---> "epemph8hv" :w = omega, 8 = theta, h = eta, v = nu, and -ph- = phi)<br /><br />(3) if it is a second aorist, the only difference between the aorist indicative and the imperfect indicative will be a slightly altered and shortened tense stem. (in the verb "leipw" the imperfect is "eleipov", and the aorist indicative is "elipov". The only difference being the change from -leip- to -lip-.<br /><br />Well, I hope this has helped rather than increased your confusion. You could try to memorise these points, but don't bother. Just be aware of them, and work on learning all principal parts of the verbs and writing out the conjugations. Soon, everything that was confusing before will become second nature. Try to internalise as much of the language as possible and become a natural reader, soon you will be able to set aside the scalpel of the forensic philologist without the need to disect and identify your forms any longer.
<br /><br />vale bene,<br />Seba<br /><br />