It is a learning curve, but here are a few hints to look for.
1). Vapor production goes way down
2). Very airy draw, more than normal.
3). The taste goes away.
If #1 or #2 happen, make sure that your battery isn't low or dead. The last few vapes on a battery will give the same symptoms.
If you get a burnt taste, you've gone too far and mot likely the carto is junk. If the carto gets a very tight draw, it may be gunked up, but make sure your battery air holes are clean first.
Luckily, smokeless image cartos don't burn nearly as easily as others. If you do burn them a little, you can usually still use them because it won't burn bad. For an explanation why, watch my video on SI cartos. It may also help you understand what you are looking for.
http://www.youtube.com/watch?v=g405PASX1NA
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