On the whole, you get what you pay for - or at least you don't get what you don't pay for! Unfortunately, if a supplier seems cheap then the quality won't be as good. There are no short cuts that will allow good quality oils to be produced cheaply. Cheaper oils may be from third or fourth distillations or from the wrong part of the plant. The oil may be mixed with cheaper oils or even synthetic oils; France actually exports significantly more lavender oil than it produces.
A second indicator is the variation of price across the range of oils. The methods used to cultivate and harvest the plants and extract the oils differ widely, and so should the prices. For example, it takes about eight million jasmine flowers to produce one kilogramme of jasmine absolute, and the flowers must be hand-picked at night when the smell is most intense, before the flowers are a day old. Examples; lavender and fennel are cheap, clary sage and frankincense medium, jasmine and neroli expensive. As a rough guide, medium would be from 5 to 10 pounds sterling. Organic oils are more expensive.
A dark bottle, the latin name of the source plant, the country of origin and the method of extraction. For some, the part of the plant; e.g. juniper berry.
The supplier should be storing the oils in a cool, dark place. Remember that essential oils are volatile and so do not have an infinite shelf-life even if kept correctly, so try to buy from somewhere with a reasonable turnover. An example of where not to buy is a small local grocers near where I live in London; it has a box of essential oil bottles taped to the top of the till, covered in dust and in direct sunlight in the window.