That's a better question than it may first appear and no where near as obvious. When in the menu for testing flipper solenoids, there's also a hold test, and to be honest, I'm not sure what that's supposed to show either as I've never owned a machine where it was actually held during the test, even one with new flippers.

Take the coil stop off and see that it is flat and not concave.

Look at the plunger, is the tip penned back (mushroomed?). Is the plunger shaft smooth and glassy?

Look at the plunger link. Is it free of play, or is it loose and sloppy?

Is the return spring broken or unevenly wound?

http://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers

My answer is:

When you just bought the game and it hasn't been shopped or when the linkage has a lot of play in it.

If I'm shopping a game, it gets a rebuild kit. Simple as that.

Even if you buy a NIB game, unfortunately the flippers wear much like the steering linkage on your car, a little bit every time you use it. You don't usually notice a difference until you replace the worn part or something breaks. One sure way to tell, the flippers will start to get weak, unresponsive, stick or drag. A simple inspection of, how much play is in the flipper, does it have any sticky spots if you work the plunger by hand, etc, will normally dictate a rebuild.

I just replaced the spring, plunger mechanism and coil sleeves on my bsd. I couldn't make the left ramp very well and I could see the linkage on the plunger had a lot of play in it. Replaced the parts and now it's like new, and at a cost of about $15 for both flippers, and 15-20 mins per side.

Unless something is broken or really dirty, flipper parts will often wear out before you notice any loss in power. Coils don't wear out. They work 100%, or not at all.

If you're looking to buy a used game, check the plunger link for play. On Sterns, the stainless allen head screw they use to attach the crank bar eventually wears down. The bushing and the link also wear, but that screw wears the most. Wiggle a new game there and you'll know how tight they should be. If there is enough play, the EOS switch actuator can get stuck on the end of the EOS switch blade. Same goes for other manufacturers. The plunger/crank bar link takes the most abuse. Excessive play there is a good indication that the flippers need to be rebuilt.

Knowing the power of different coils is also helpful. Many WMS games use 11629 or 11630 flipper coils. 11629's are way stronger than 11630's. If you see 11629's in the game, but they play like 11630's, something isn't right.

If the plunger or coil stop are mushroomed - rebuild.
If there is binding and they are not overtighted - rebuild
If they are so damn dirty the only color you see is black - rebuild
If the flippers drag on the playfield or the playfield bushing is cracked - most definitely rebuild

My recommendation is if you are a new to the hobby you should fully rebuild a set of flippers with all new parts, you will probably end up replacing some parts that don't need to be replaced but better to replace everything.

Follow Vid's guide for how to correctly do this.

Now can now use these newly rebuilt flippers as a baseline for what a really good flipper feels/looks like. Try to keep this in your mind as a reference and you can compare with other pins. The more time you work on pins and play on them you will get a good feeling for when the flippers need work and what needs work.

It is not just about power, but how "snappy" the are (fast to go up and return without drag or spots of hesitation) and how tight the flipper is (is the bushing blown out and the whole flipper is rocking around).

In a home use only environment it will take a long time for your flippers to wear out if you fully rebuilt them. Probably the most important thing to do to make your home pins more enjoyable. I don't worry about a few lights being out, but flippers that are not good demand immediate attention.

Where is the best place to buy flipper rebuild parts? Shows seem to give the best prices but are we talking kits from our online parts sources like Pinball Life and Marco inbetween if we need?

It depends on what the game is. Some games (can we say Bally linear flippers?) cost much more to rebuild. Just make sure to get the bushings for the flippers, even if they aren't included in the kit.

Mad and some others have kits as well as pbresource.

I get mine from pinballlife.com. You can get all the individual parts, very inexpensive. Shipping is often the biggest cost of the order.