:sl:
A brother wrote this up a while ago, it should asnwer your question Insha'Allaah:
The classical Sunni scholar Imam As-Suyuti cites in his monumental book on the sciences of the Qur’an al-Itqan fi Ulum al-Qur’an fifteen or so characteristics of the Mufassir. Scholars affirm that any tafsir (commentary), which disregards these principles, must be scrutinized with great concern and caution, if not completely rejected. Below are some important conditions:
1- Proper intention and sound creed.
2- Knowledge of the Arabic Language; this requires one to master Grammar [nahu], Morphology [sarf], word etymology [ishtiqaq], Arabic rhetoric [balagha], Poetry amongst other things.
3- Knowledge of the various modes of Qira’ats [recitations].
4- Knowledge of the Principles of Fiqh [Usul al-Fiqh] and Fiqh.
5- Knowledge of the Asbab an Nuzul [reasons for revelation] and its related topics.
6- Knowledge of the Nasikh and Mansukh.
7- Knowledge of Hadith especially those pertaining the explicit commentary made by the Prophet (pbuh).
8- Knowledge of the makki, madani, muhkam, mutashabih and the various types of ‘Ijaz of the Qur’an.
9- Referring to the reports of the Companions of the Prophet.
10- Considering the reports of the successors of the Companions.
11- Consulting the opinions of eminent scholars.
12- Following the proper methodology of exegesis of the Qur’an.