Kaal Sarp dosha is said to form when all seven planets from the Sun to Saturn sit on one side of the line from Rahu to Ketu. It is checked here by exact degree, not just by sign.
About this page
What this page tells you. Whether all seven planets fall within the half of the sky from Rahu to Ketu (or from Ketu to Rahu), and if so which of the twelve named kinds it is.
How to read it. Enter the birth details. The answer is at the top; the table lists every planet's exact position so you can see the line yourself.
Good to know: A planet sitting just across the line breaks the pattern; the page says how many planets are on each side. Many astrologers do not use this dosha at all.