Talmud for Menachot 84:15
ת"ר תכלת אין לה בדיקה ואין נקחית אלא מן המומחה תפילין יש להם בדיקה ואין ניקחין אלא מן המומחה ספרים ומזוזות יש להן בדיקה וניקחין מכל אדם
Our Rabbis taught: There is no manner of testing the blue thread;<span class="x" onmousemove="('comment',' To ascertain whether it has been dyed with genuine or imitation blue.');"><sup>24</sup></span> it should therefore be bought only from an expert.<span class="x" onmousemove="('comment',' One who knows that vegetable blue or any other imitation blue is unfit for the purpose.');"><sup>25</sup></span> The tefillin can be tested,<span class="x" onmousemove="('comment',' To ascertain whether they have been made according to prescribed law and whether the Scriptural portions therein have been correctly written.');"><sup>26</sup></span>