To know for sure you have to order GCMS notes to see what the officer is writing about your application. They will write comments such as, "Couple has many pics, but do not look compatible", "Sponsor visited spouse many times over the past 2 years", "Both parents attended the wedding, as well as their extended families", etc.
Red flags are things that are viewed negatively in an application, such as:
-couple has large age difference
-large educational, cultural difference
-getting married quickly after meeting
-no parents from either side attending the wedding
-few pictures and proof of communication
-couple do not speak common language
-applicant has many family members living in Canada, but not so much in home country, which is viewed as having a strong desire to immigrate to Canada
-applicant has shown numerous attempts to enter Canada