99 Infiniti QX4 HELP!!!?

We just bought a 1999 Infinity Qx4 about a month ago with 155K miles on it. From a private owner that did not know the whole maint history for the vehicle ( I know I know red flag ). We have take it to a mechanic that told us we should change the timing belt to be safe incase it wasnt done at sched mait time. We now have a little over 156 k miles and its running fine. I dont want to spend $500 to change something that could already have been done and throw money away. Any suggestions? I have tried to get someone to just check the belt and its the same price to just change it cause they have to open up the engine. What shoud I do?