Real good hash must bubble when touched by the flame or it turns to oil immediately (but thats more the case with concentrates). The more it bubbles the better it is usually. But some hash sorts don´t bubble usually like Red Lebenon... for me that is more hash only for the taste or maybe for that very light stoned but if you want potent hash it usually has to bubble.
But like you said some bad people add some things to make it bubble so you can´t say that a hash is guaranteed of good quality only by looking at how it bubbles. You have to inspect the whole product... first of all a good hash has usually a pleasant smell no chemical scents or something like that. No smell is also a not so good sign but some hashes begin to smell when you warm it in your hands for a few minutes...
The next thing is how the smoke looks and smells like when burned a small piece... black smoke is usually a bad sign also chemical scents... one time I did that with a hash from Maastricht for 15€/g! and some parts of it acts like when you burn plastic... there were small burned pieces flying through the coffeeshop hard to describe in english for me only saw that one time again with a batch of Waterpressed Afghan from Bluebird
Overall it´s not so easy with hash to say if it´s 100% clean. You just can inspect your hash (bubble/smell/smoke/taste) and buy from sources you trust and when you smoked more and more hash you also got some experience in what is good and what maybe not... but the hash "industry" changed very hard in the last years... so it´s hard to be up to date