For the engine you just need to take a look at your cars power output, if it's started to drop from what it was and the oil changes won't take it all the way back up, then your due an engine re-build. The chassis refresh is harder to work out as there's no on screen data to provide you with any indication, you can only go with how the car feels. I have noticed on some high mileage used cars that I've bought that after a couple of races if I restored the chassis then there was a noticable improvment in the cars cornering ability and overall stability. I don't think it's a placebo affect as this was also a feature in GT4, but I don't think it needs to be done very often. Mainly I think this will only affect certain used cars you buy and cars you tend to use very often and even then you'll only need to restore the chassis occasionally.