What are the early warning signs that indicates that your car's starter needs to be changed?

There don't tend to be warning signs. The starter either works of it doesn't.

More that likely it's the battery.

what others have said, battery or dirty battery terminal connections are the MOST likely reason, but many a mechanic have sold starters when all was needed was a terminal being cleaned.

Where do you get the idea that anything gives an "early warning" sign of failure?

The first time it acts odd or won't start is the warning sign.

Battery, battery cables, corrosion on battery posts/terminals, damaged/worn solenoid contacts can give the symptom that you're having. How old is your battery?

The actual voltage at the starter and the amount of current it is drawing when this happens would indicate something.

I turn headlights on and see if they dim a little or a lot or go out, when starter/starting problems occur.

-Engine overhaul mechanic and general automotive mechanic since 1972

Take it by an auto parts store, they can check everything and tell you what it might be. FREE.

Starter, Alternator & Battery are 3 possible things. Cable ends are another.

If your battery is 3 years old or older may be time to change out for a new one.

Take your car to an O'Reileys and have them run a diagnostic. They will tell you if your started fails their test. They told me it was ethier the started itself it a bad connection. I recently had my started replaced, and it helped my car to start more crisply and it is no longer sluggish and slow. However, it still takes longer than it needs to so it could be a bad electrical connection (corroded wires)

if it tells you

Could be a low battery indication.

I would start with battery .

No.
Starters rarely give notice to quit.

Could be a simple case of a poor electrical connection.

I'd suspect the battery if it's the original. It can be tested. That starter should last another 5-10 years.

When the car won't start

Starters actually rarely fail. The problem is far more likely to be electrical, maybe battery, cables or connections.

Sounds like an old battery or dirty terminals.

getting a clicking noise at any time means it neesd to be replaced or you will just get stranded at some point. there are places that can check the electrical and tell you what needs to be replaced. usually at no cost.

It clicks

w/o a test you cannot know if this is the battery or the starter

Sometimes, none. Not engaging bendix gear or slow cranking or not working, sometime, can be yoke, brushes, and/or solenoid.Slow or no crank hot when it started good cold shows the starter may be getting too much exhaust heat. This can cause failure.

BATTERY low voltage

There are various signs. Slow uneven cranking can be the bendix drive, or weak battery. If the thing stops doing anything (solenoid problem) or just clicking. Most of the time you can revive a non-functioning starter by whacking it. But be careful you could break the case.
It sounds in your case like anything. Dirty connections, weak battery, maybe weak alternator. You can probably get all this adequately tested for nothing, if you take it to the right place. There is also sometimes a replaceable bearing the starter runs in, but more likely it is enclosed.

I would think it is more an indication of the electric energy that the battery delivers to the starter; the battery may be weak or the cables from battery through the various cables and contacts may be an issue.

Who knows

weak battery, corroded battery wires, but likely the starter has a bad spot... get it rebuilt instead of a reman unit...

Weak battery

thats common where we have a solenoid that has corrosion on assert contacts . if we have external ( not on starter ) solenoid .thats our suspect at first .but we may also have other issues.do check contacts .battery terminals etc for corrosion. looseness etc
if we keep getting odd slow cranking at first ( used to be called a lazy starter) we really ought to exchange to avoid issues. winter on way ..

Yep

Noxious fumes

First and foremost make sure the terminals are clean and tight.(something like a teen hooker).Especially the - negative terminal.Clean,tighten,then apply a good gob of "vaseline" (Homo Lube) to each one to keep clean and dry.

i have no car

Not at all; You should/will need a new car that runs on 10% Ethanol ONLY. SUPPORT YOUR CORN FARMERS - GO NEBRASKA CORNHUSKERS! GOOD WUCK!!!

There are three major causes of starter failure:
1. Burnt windings
2, Worn bearing surfaces
3. Worn bushings
All of these can be tested, but to test them the starter must be removed from the car.
There is also a pair of contacts in what is called the solenoid which can fail, with few exceptions these are part of the starter.
The one other part which fails is called the "bendix", if it fails often the starter will just spin with out engaging the flywheel.
Turning slowly can be an indicator of starter failure. Keep in mind ambient air temperature affects how well a battery operates. Cold temperatures also thicken the oil and other fluids making the engine turn slower. Another factor is (in the case of a manual transmission) how well the clutch release system is working.
Starters now often last the design life of the car (100,000 miles or more), but batteries only five or less years.
The first thing to have tested is the cars charging system and the battery. Many parts stores will test these in the car for free. If the charging system and battery check out alright then you have to decide if you want to wait till the starter (or one of the other parts that often are replaced as a unit because they are in a new starter) fails or just get it replaced so you can not have to worry about being stranded or getting a tow.

The car will not start.