What is a real master

A real master inspires the trust in the person who has the urge to surrender, to do just that: surrender, and does consider this gift precious enough to refrain from anything that would seem incompatible with that trust. He respects the agreements made explicitly and even the agreements that might have been made, had the submissive thought of them. He takes responsibility for his own acts and for those of his submissive, but demands that the submissive embraces her responsibilities accordingly.

Just sex" and abuse are two things that either can or cannot be part of a D/s dynamic. If a submissive or slave craves such things, and that is the case more often than you might think, then by all means she should search a Master who is sadistic or selfish enough to provide that, according to his nature. Everything that works for two people is OK, if they are in a position to make autonomous choices.
You have to make the distinction yourself, its your responsibility and no one elses. For someone inexperienced, it may be very difficult to spot the many predators who find a happy hunting ground in the lifestyle, with many gullible submissives just wayting to be exploited. Read whatever you can on the many bulletin boards, ask advice, specially about safety measures that minimize the risk of abuse. Things like safety calls, like not exposing yourself the sphysical control of a Dom without solid identification by which he can be traced, in the hands of a trusted one, etc. Protocols exist; use them. Trust needs time; take it. Submissives that expect to be collared within a week on the site (and often are!) are not always realistic and hardly a good example to their peers.

Being hurt/abused before, makes you aware of the dangers, as well as make you too anxious to recognise a Master and open up to hem, if you meet one. The remedy might be a repeated exposure to the "substance" you are allergic to, in a controled, safe environment, as is done in some forms of allergy management therapy. All it takes is someone who is a friend, but not interested to exert power over you, to accompany you to 'scene' environments. There you can learn to discern without risk, to learn how to recognise signals; in short to gain the indispensible experience.

These two extremes should be easy to tell apart. It's the ones that are in the grey area in between that give you a hard time. Once again, what you need is experience without the damage that sometimes seems inevitable in the process of gaining it. But it can be done. Find a tutor, a mentor, but -and there's the catch- you find the biggest predatores in that category too: the inexperienced preys come drifting your way. One way to help you make a choice is to ask for ample proof of identity, and understanding for your needs for safety measures at the start. If he shows the least hesitation, that's a yellow or even a red flag.