Well first, you should try to be able to translate a word in your L2 into the correct word in your L1 to prove your fluency and mastery of the word. This is critical in inferring the definition of a word.
Well first, look at lots of words surrounding the unknown word, not just the ones in that particular sentence. By looking at lots of words, you get a better sense of the usage of the word. For example, let's use
emergency as your mystery word here. It has been seen in this example text:
You see that it is an emergency that needs to be reported!
Of course, through translation, this sentence would practically have anything in place of emergency. So let's try to expand the search area for about two-three sentences surrounding the sentence containing the mystery word:
You walk down the street, sipping on your freshly brewed cup of coffee. You then hear a piercing scream coming from the alleyway in front of you. Peering into the alleyway, a man can be found on the floor in a pool of his own blood, unresponsive. You see that it is an emergency that needs to be reported! You reach for your cell phone, adrenaline rushing through your blood. You dial 911 and quickly discuss with the operator about the situation. Seconds later, you hear the reassuring sound of sirens and see the flashing red and blue lights.
I understand the above text is poorly written but it should help. From the text, you should try to find words used for a similar mood, feeling, or some way. Words that invoke a lack of security or a need of one is used: piercing scream, unresponsive, pool of blood, exclamation mark, adrenaline, etc. You now understand that emergency is used during a time of urgency that requires immediate action due to an unexpected and dangerous situation.
Next, how is it used? As a noun, verb, adjective, or something else? Since it is used after an article (an) and there is no noun after it, it surely has to be a noun. Now you got a good prediction of the actual definition:
a serious, unexpected, and often dangerous situation requiring immediate action (Google)
So let's recap:
Read surrounding text for clues
Look for word trends and identify them
Determine what the word could mean based on the trends
Find its part of speech
Infer its actual definition based on the results of step 3 and 4