Look them up in a dictionary. You can guess what a word might mean if you recognize its root and part of speech, but that would just be a guess.
It would be nice if there were some simple rules that you could say, "any word ending with -or means the person doing the thing", like "auditor" = "a person who audits", "creator" = "a person who creates", etc. But English isn't that simple, and I'd guess most other languages aren't either. A "janitor" is not a person who "janits", nor is a "boar" a person who "boes". Etc.
In many contexts, the meanings are pretty much the same, but you might hear finished more often than completed in casual conversation. For example:
I've finished my shopping.
She finished the song.
He finished the race.
I could use completed in those sentences – the meaning wouldn't change, but the register might sound off.
The word completed can convey some sense of accomplishment. In the context of a race, it might work when the race is a major achievement:
He completed his first marathon last year.
Homework, though, is not really a major achievement, so I think you'd hear finished more often in casual conversation:
“Joey, where are you going? Did you finish your homework?”
That said, you might see completed in more formal contexts, such as a paper on education, or a course syllabus:
Students must complete six homework assignments during the semester.
Best Answer
You can't replace the adjective 'likely' with 'possible' in your sentence:
'It is important to know how to identify what a reference word refers to. Some questions in the exam are likely to test this ability'.
You would have to say '...some questions in the exam may test this ability.' This is because 'possible' is not a verb.
That said,
'Likely' has a greater chance of occurring than 'possible' (or 'may').
In fact 'likely' means it has about 50% chance of happening, whereas 'possible' or 'may' gives no indication of probability at all.
Examples:
https://en.m.wikipedia.org/wiki/Words_of_estimative_probability