How do i know that i would need to change my spark plugs?

what are some signs that i would need to have my spark plugs changed without actually getting under the hood and looking at them?How do i know that i would need to change my spark plugs?
IF it's been awhile or if the MPG are going down and the car just doesnt seem very strong.How do i know that i would need to change my spark plugs?
You should change them about every 40,000 miles... I know the dealers claim you can go 100,000 without changing them but at that point they are in real bad shape and REALLY tough to get out! If your gas mileage goes down, hard starting, or idle gets a bit shaky that is a sign as well.
I would just pull 1 out and look at it, you don't have to check everyone. If its been over 20,000 miles, they may be due anyway
your engine seems like it running hard i had one that came out and it hit the top of the hood everytime I drove.. ,, bad gas mileage.. it could be several things.
black smoke from the pipe and the engine is running realy rough like just not normal. also just pull one out, the way it looks should tell you if you need new ones
hard to start.....poor running....old age.....high milage.