When we burn candles, we often watch and wait for divinatory signs that tell us how the work is going to come out -- that is, whether the spell will be a success or not. Some of the common signs we observe are so-called "coincidences" (especially names and subject matter that relate to those in the spell). We can also consult a system of divination, such as using a pendulum or a Jack Ball, reading or cutting playing cards or tarot cards, or employing Bibliomancy (divination by means of a book such as the Bible). Another easy way to get a divination on candle-burning spells is through ceromancy -- divination by wax. In this case, the wax we "read" is the wax of the candles themselves.
Some signs one may wish to interpret according to the ritual work he or she is doing:
The candle gives a clean, even burn.
The flame flares, dips, gutters, and flares again, repeatedly.
The flame hisses, sizzles, pops, or makes other noises.
A free-standing candle runs and melts a lot while burning.
A free-standing candle burns down to a puddle of wax.
A glass encased candle burns half clean and half dirty.
A free-standing candle lets out a lot of smoke but burns clean at the end.
There is a dirty, black, sooty burn (especially one that messes up a glass encased candle)
This means things.
A glass encased vigil candle cracks or breaks, spilling wax.
The candle goes out before completely burning.
The candle tips over and flames up into a fire hazard.
The candle burns up overly fast.
For a more in depth analysis of the points listed above, see the Catherine's site at the link above.