Some laptop keys do not work

For the past few months I have noticed that a few of the keys on my HP Pavilion N5195 laptop do not work when I am typing. It seems intermittent, in that sometimes one may not work and at other times it works and a completely different one will notwork. I attached an external USB keyboard and have no problems at all so I’m assuming it is a problem with the laptop’s keyboard. Is there any other test I can do to pinpoint the cause of this problem
C.M.Stewart