You can't tell exactly how much life is left in a starter but you can tell if it doesn't have long to live.

If you have a 2 channel scope you can measure battery voltage (at the positive and negative battery cables) and the Starter Amp draw (at the cable going from the battery to the starter (using an amp clamp)).

So in the 2 screenshots, the 1st is a healthy starter. I added some white lines (and some "Event Point" text) so you can see how the battery voltage and starter amperage draw are pretty consistent after the starter starts turning the engine.

The Yellow line is the battery voltage.

The Green line is the starter amps.

GOOD:

D:

This 2nd image is an unhealthy starter. I added some white lines so you can see the amperage draw dropping to zero (same as Key "OFF") during the starter's revolution and the corresponding battery voltage recovery when the starter amps drop off.

This starter has a damaged/weak/dead spot/short on the commutator/armature.

(so even though it's impossible to know exactly "how much life it has left", this starter is worn/damaged and is going to fail)

BAD (amps dropping to zero):

Not that I know of.

Never in my life have I had a starter fail on me

There is a tool to test if a starter is good or not but I don't know if there is a tool that can tell you the lifespan of the starter.

I'm not aware of any...it either works or it doesn't. Back in the day sometimes you had to hit it (long pipe or long 2x4) to help knock it just a bit and would kick in. The issues with some modern vehicles is they're susceptible to heat soak from proximity to extreme heat. 

Not that I know of, although, I remember Scotty said that he put a starter in the trunk so if it goes out, he can change it.