A few questions to get some of the right answers:
1) When did this start happening? Has it always been this way, or did it happen suddenly?
2) If it used to work better, can you think of anything that may have changed between when it was faster and when it wasn't? I.e. did you let it sit for 2 years, did you recently do maintenance to it? Did you lend it to your son for the night? Etc? Also, does the gas pedal seem to rest at the same place it used to?
3) Do you have more details about what kind of engine/model/etc it is? If you don't know what engine it is, a picture of the engine bay may help people identify it for you.
4) Are there other effects that you can notice? Does the car backfire, smoke, keep running after you turn it off, make other loud noises? If so, these might suggest mechanical issues that cause your speed issues.
5) If you suspect an issue with the throttle cable, perhaps a picture or description of the linkage that you can see would be useful. (i.e. if the cable recently came loose, it may be kinked at the old adjustment location, or may now be right at the end of the cable, etc).
And for your specific question, you can tell if the throttle adjustment is correct by having someone press the gas pedal to the floor, and you can go look in the engine bay to see if the throttle lever is at or near it's max position. If you're not sure where that is, have the person at the gas pedal fan the pedal a few times, and you'll see the cable moving and pulling the carburetor's throttle lever.
And, as people have mentioned before, make sure your car is in good order otherwise -- this is a good time to check the oil level.
