In general no
A witness does not need to see the content of your Last Will, only witness your signature on it. Inheritance and succession laws in Thailand are codified in book 6 of the civil and commercial code. You can find the answers to questions related to subjects such as the content and witnesses of a last will, about heirship, disinheritance, renuncation of an estate, statutory right of inheritance, division into portions between several classes and degrees of statutory heirs, wills, forms of wills, effects and Interpretation of wills, revocation and lapse of a will or clause in a will, nullity of a will or clause, administration of an estate, realization of assets, payment of debts and distribution of an estate (to name a few). See reference link below for the full law text related to inheritance.
The answer to your question with reference to the specific section in the law:
Section 1668 'unless otherwise provided by law, the testator need not disclose to the witness the contents of his will'.
มาตรา 1668 ผู้ทำพินัยกรรมไม่จำเป็นต้องเปิดเผยข้อความในพินัยกรรมนั้นให้ พยานทราบ เว้นแต่กฎหมายจะได้ระบุไว้เป็นอย่างอื่น
