Short answer
Look at the sources: if a practice is found in the Quran or the authentic Sunnah, it is Islamic. If it comes from local tradition, for example honour killings, forced marriage, banning girls' schooling or certain dress, it is often cultural and may be against Islam.
The fuller answer
Many things that are criticised in Muslim societies come from tribal and local customs. Honour killing has no basis in Islam, which forbids killing and false accusations. Forced marriage is invalid. Education is a duty on all Muslims. Women's right to inheritance is often denied by custom, though the Quran guarantees it.
Muslim scholars often say that the best way to judge a faith is by its texts and principles, and not by the actions of those who claim it. That is also why you will find that Muslims from different regions can differ in food, clothing, wedding customs and festivals, and all still are Muslims.
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.