I was diagnosed with niche on my caesarean scar which was seen when they did the ultra sound. is this life threatening and does it need surgery? is ultra sound the only way to check this?

If there is an open area ( "niche" ?) in the C/S scar, it is a probable small incisional hernia. If there is no protrusion, it is probably stable and needs no surgical correction presently. Another pregnancy may enlarge the defect, but assuming you will receive another C/S , if again pregnant, nothing needs to be done now unless the defect enlarges.