This particular part of candle magick can be described as moderately difficult. I can't really put up a video of what exactly to look for, but I'll see if I can find something to at least demonstrate what to look for.
Now, flame reading, or ceromancy is used to basically read how a flame is reacting or burning during a spell. There is another form called lychnomancy where you literally gaze into the flame or a fire and use it to scry. I have done this successfully, but again it's not something I can demonstrate, so I'll try my best to describe it in full.
First and foremost, make sure where you are burning your candle is safe and does not have any form of wind or air blowing. In this case, I would even suggest turning off any fans in the room and maybe even closing vents depending on how high your air conditioner is and how strong it pushes the air out. Using a glass or encased candle won't make much of a difference in how it burns or on how to read a flame. However, if you plan on reading the wax, using a free standing candle is preferable. Always make sure you're doing the spell on the right day, at the right time (if possible), and for the right amount of time. A longer burning candle is used to for long term results or to get over more complex issues. A smaller burning candle, like votives or tea lights, are for quick and easy remedies or prayers.
Throughout this post we will mention blockages, these can be from outside sources or it can be you, yourself, sabotaging the work. Always look at the other signs to correctly read this so you can address the issue the right way.
Reading the Flame
If you're trying to split a couple up , you usually use either a duel wicked candle, like a married couple, or two separate figure candles. As they burn and both flames continue to burn throughout, then they won't separate. If one goes out or burns down first, then separation is likely.
Now in some cases of attracting a lover you'll use just one candle; mainly for general love work and not a specific person or trying to repair a split or current couple. If you're burning a candle and a flame comes out of the wax then the spell worked. If the two flames meet and become one again, then it'll be soon.
Another way to read this is if that single flame splits itself off into it's own, meaning going away instead of coming in. This would mean another person is involved or there's a spirit near by. Now, this can depend one 1) the type of spell work you're doing and 2) which type of candle you're using. For instance, if you're trying to contact the other side, then something is present and trying to reach you. If you're using two candles for a love spell, this could indicate an affair or interest in another person. It can also indicate a separate issue arising. Use your own judgement and insight when this happens.
Sometimes when you do a spell for other people or even yourself you'll hear a crackle or pop sound. This means that a spirit is trying to communicate with you or that there's some insight about the situation you need to know. I usually pull out my tarot and do quick readings to see where the conversation leads. You can also scry at this time using the actual flame since that's what they're communicating though or any other form of spirit communication you're comfortable with. There are also cases of it being the person you're doing a spell for is talking about you in some way. This would depend on the spell you're using and also any other signs the flame or candle is giving off.
If the flame flickers and you're using a devotional or a request from the Gods type of spell candle, then your petitions to the Gods will be heard. If you're trying to contact the other side for any reason, this is also a sign that spirits are near.
If the wick starts to build up or form a ball, there is some kind of blockage. Watch to see if this build up follows the wick down and how long it takes before its burned through. If the candle burns out and it's still there, then you need to try another way to first overcome or deal with this person or issue.
Length of the flame and color is also important. A nice, tall flame indicates that there is power behind the spell. If it is low or barely burning, you'll probably need a cleansing or there's a bloackage. If it's extra tall and thin, but doesn't produce smoke or soot, then you're in the presence of a benevolent spirit. If the flame color seems a little dark, or the flame appears to be acting aggressively, then the spirit is darker, but that doesn't mean it's there for an evil purpose. In both instances they're there to assist you. If the core is a normal healthy color (red center, slight blue lining, and a yellow outside), then the Gods are working are diligently. If the red core seems bigger, then the Gods are already at the heart of the matter. A dim core means that the motivations are not necessarily truthful. If there's a large blue lining, the outcome may not be favorable.
Reading Encased Candles
However(!), if you're burning a candle and it cracks without there being any evidence of the candle being made wrong and the glass is indeed heat safe, then that means there was blockage. If it doesn't break off, shatter, or chip, then the spell was protected and worked. I would still not continue using this candle. If the glass does break, then the blockage did stop your spell from working. You'll either need something stronger or to first work a spell to protect yourself and end any work being done against you or your services.