how would i know when to change the spark plugs?

Spark plugs seem to last longer in newer vehicles. Haven't had to change many until after 100,000 miles. No misfires, stumbling from engine, probably not. More my opinion of air restriction, or fuel restriction. Check air filter, make sure light is good looking through it. No exhaust restrictions, fuel pressure and fuel volume. Easier for me to run data on computer to be sure. Of course could be different problem all together. Have it tested to be sure.