Rephrase question. If I can rephrase your question- you are asking how to check skin cancer if your skin is full with tattoo? The answer is-- it would be difficult. Your dermatologist really have to look very carefully to see if there is anything suspicious. You have to pay attention to your own body, if you feel a growth or something not right -bring that up to your doctor. And, dont add anymore tattoo, if you can.
Need more info. What exactly is your question?