Well light never overtakes light. But if you are on a ship travelling at 99% of light speed and turn on the ships front light, you would see that light move away from your ship at the speed of light relative to you, i.e. it would look like it was moving away from you at 300,000 km/s, from your point of view.

But would that mean it was travelling faster than light from somebody else's viewpoint? The answer is no. Light never overtakes light. So what would someone else see?

They would see your ship travelling at 99% of the speed of light, and that light would be moving at the speed of light, so they would see the light moving away from the front of your ship, moving 1% of the speed of light faster than your ship is! They see the light moving at c, and your ship travelling at 99% of c.

So what is happening here? How can anyone, whatever speed they are travelling at, see light move 300,000 km/s faster than themselves?

Well, if light is the constant, then it must be distance and time which are changing depending on the viewpoint. Your rulers and your clocks measure the universe differently, depending on your velocity relative to somebody else's rulers and clocks.

We have found that clocks tick at different rates when they travel at different relative speeds. This is unnoticeable in daily life, but we have tested this by putting very accurate atomic clocks on jet planes and flying them round the world and then comparing them to a clock that stayed on the ground. It seems that when you travel relative to another object, your clock ticks at a different rate relative to that objects clock.

Another effect of different relative speeds is a difference in the way you measure the universe around you. As speeds increase, objects seem to shrink in the direction of travel.

You asked how we can tell how fast we are going, but you answered the question in the same sentence - "I mean if you are on a bus that is going 20 MPH and you walk forward 3 MPH are you going 3 MPH or 23 MPH?". You can only judge your speed relative to another observer. You are walking forward at 3mph relative to a passenger sitting on the bus, but you are moving at 23mph relative to a person standing on the ground watching the bus drive past.

But who is to say who is moving relative to who? If two ships pass each other in deep space, who is to say which is moving and which is not? Well it turns out that either ship can think of themselves as being at rest and that the other ship is the one that is moving, and they would both observe the same amount of change in the length and the clock of the other and would both measure light as travelling 300,000 km/s faster than themselves!

One way to think of it is that the closer you approach the speed of light relative to another object, the bigger the difference there is in the rate that your clocks tick and the length of your rulers, which means you will always measure the speed the light as the same figure, and it is your perception of space and time that change for you both to make up the difference. And it works the same for everyone, whatever their relative speed.