1. Go to www.baheth.info
2. Type in the required word without harkaat using the yamli enabled search box given on the site, or copy paste after typing in eiktub.
3. After searching, it displays the word highlighted in all entries from the books 1st giving the more relevant entries usually.
4. Suppose you want to know the root for the word قُدُّوس
5. You type the word using yamli on same site or type in eiktub as قدوس and then paste in search box and search.
6. On searching you find the results in all 5 books giving all entries containing this or similar word with different harkaat. See below screenshot. 1st 4 results clearly indicate that its root is ق د س and to make sure further, see whether the words are used in قُدُّوس and you find out that they are used.