Replace the current fans...especially if they are "old".
Fans/heatsinks are not very expensive...compared to the value they provide and the time it takes to replace/install.
As for the question posed in your title...you can't directly, since such crashes provide no error messages. You can check CPU temperatures by going to the BIOS and reading the max operating temp for that CPU (they are not all the same, may vary by 20 degrees Celsius or so).
I've had this program recommended by those I trust, Core Temp - http://www.alcpu.com/CoreTemp/
How to Avoid Overheating Hardware Secrets - http://www.hardwaresecrets.com/article/142
CPU maximum temperatures - http://www.pantherproducts.co.uk/Articles/...peratures.shtml