How can you tell when and Russian letter о is reduced/shortened to an а?

For example, отлично is pronounced as atleechna and Прекрасно is pronounced as Prikrasna.

When you look at a Russian word that contains the letter о, are there any rules or maybe general rules that you can use to tell when an o should be shortened to an a just by looking at a word?

When you come across a new Russian word it is always worth checking where the stress is. Unfortunately, stress marks are not shown in authentic Russian publications (except for Russian learners textbooks). However, Russian dictionaries (as well as wikipedia) DO show them. 
The letter o always undergoes reduction when the stress doesn't fall on it in a word. Other letters can also undergo reductions, please check them in section 1.6

Thank you.