Try to stick to essential oils and avoid scented oils (there is a difference), perfumes, scented candles, and even incense. There are some recent studies showing some adverse effects they can have (like asthma, lung damage, and tumours). Air fresheners have formaldehyde.

The label should say pure. Sometimes an oil may be adulterated (diluted with carrier oil). These are generally cheaper. I have a few, they just don't last as long though as I have to use more.

Young Living and Doterra are good recognized brands available over the internet but I have also found some I am happy with at my local health food store for a fraction of the price. Again look for the words 'pure' and 'therapeutic grade' on the lable.