What are some symptoms of Spark Plugs being due for a change?

One approach to the question could be, if it is running well now, don't obsess over them; if it later develops any misfiring, consider the unknown-age plugs to be reasonable suspects.

Another approach might be ... maybe you'd like to get leakdown measurements somewhere around this mileage, just so you will have them to compare to later ones and track how well the engine is holding up. If you do that, you'll be taking the plugs out anyway, and with the wrenching involved, it's just as easy to put new ones in.

 

plug your vin into the toyota owners site and see if it shows any maintenance records.

without that knowledge, when i used to buy used cars, i would do all the required maintenance for the mileage.
for plugs, i would at the very least pull them for an inspection. it's good to keep the threads exercised

 

Plugs can last a crazy long time now as they are self cleaning. If they are hard to get to, you often can let it go until you have good reason to access them.

 

Or if you're mechanically inclined, spend $30 bucks and replace them when you're bored one weekend. Throw in an air filter, cabin filter and maybe even an oil change and transaxle fluid change. Then you'll know. A buck 25 or so and you'll have complete peace of mind that the car is good to go for quite a while.......

 

don't forget your egr circuit!