It is always good to be mindful of potential commercial frauds regardless of the product for which you are shopping. There are no hard and fast rules on determining if a particular tarot reader has legitimate skill or if they are simply perpetuating a fraud in order to make money. Part of this has to do with the nature of tarot reading: it does not produce a measurable effect, but rather advice concerning future possibilities.
There are, however, some general issues for which to look out and evaluate:
Does The Reader Only Provide Optimistic Predictions?Tarot reading is the evaluation of influences at work on you and/or in conjunction with a particular issue. It's reasonable to assume that not everything in your life will be butterflies and roses. If you reading sounds too good to be true, be warned. To readers this is a profession, and repeat customers are their bread and butter. Some, therefore, are more interested in telling a customer what he wants to hear rather than what all a reading actually indicates.
Does The Reader Attempt To Sell "Solutions" To Predicted Problems?Many readers are located in new age stores that sell a variety of magical accouterments. There is nothing wrong in this in and of itself. However, be extremely wary if a reader warns you of something dire and then offers to work a spell for you (for a price) or sell you items that will help protect you specifically from the predicted threat.
Such behavior strongly suggests that the reader is using the tarot reading primarily as a means to market other products. Worse, he is generating fear in the attempt to make his products more appealing.