How do you know the kw head gasket repaired?

my vauxhall corsa 1.2 x reg , gasket is on the way i was told an i flashed it out and but some k-sealer should i leave it or fix it as it would cost me £250pouns. or try out the k-sealer please any one who can advise me wlc , many thanks

The only permanent fix for a blown head gasket is to replace it with a new one. Anything else is just a temporary emergency repair.