This answer is going to assume that your question ultimately expands to be:
How do I tell the difference between the ordinal and the unit of time?
Both the ordinal and the unit of time can be referred to using either article (the definite the or the indefinite a), so that's not going to help.
Commonly, when referring to the ordinal second, you'll see it describing something else:
- The second car has crossed the finished line.
- A second racer has crashed into the wall!
- The competitor must score a second perfect to advance...
- This is now the second time that the racer has earned second place.
Adding to the confusion, the ordinal can refer to "prize" or "place" without adding those additional words and still be valid English:
- The second prize goes to Nutley.
- Second goes to Nutley.
- James lost to King, earning a second place finish.
- James lost to King, earning second.
However, you'll commonly see ordinals with other ordinals:
- I had no idea the car could have leapt from fifth to second!
- That puts Mary in first, Jesse in second, and King in third...
- First, we break into the bank. Second, we loot the bank. Third, we all get arrested...
When second is being used as a unit of time, they'll have a context relating to time:
- I'll be with you in a second.
- I'll fire the second I see it move.
- The second the item goes on sale, I'm buying it.
- A second isn't much time to react.
Just to reiterate, the context is what will tell you one way or the other, and why writing grammatically correct English can be very important. Take the following terse sentence, do you know what they mean?
That's the second second! Wait, do we have a second second too?
- Is the speaker talking about about two seconds of time?
- Is the speaker incredulous at a competitor earning two second-place finishes?
- Is the speaker confused about there being more than one person earning second-place?
Without that context you just don't know!