What model is your laptop? Does it have heat sensors on the the motherboard? If it does then try using a monitor program that will tell you the temperatures when the system is running. That will tell you if the system is running hot compared to the normal operating temperatures. I would suggest googling your laptop model and 'temperature monitoring" to see what comes up. There may be some good "how to" posts that will walk you through the process and tell you what temperatures others are getting. Most modern processors can run at 60C.
When you opened it, did you blow it out with compressed air to get all the dust out of the case? Dust build up can prevent proper cooling.