I have an easy road gear: 45 -18. That's 45 teeth on the front, 18 on the back. This is about 18mph at 90rpm. A typical fixed gear often comes with a 48 - 16. It's faster on the flats, harder to climb the hills, and is about 21mph at 90rpm.
45 / 18 = 2.5
48 / 16 = 3.0
So count the teeth on your bike. The cogs in the back will usually have the number of teeth stamped on them. Pick a pair, and do the division to get the ratio. So a 39 / 16 is 2.43 and 39 / 15 is 2.6 which are similar to a 45 -18 for instance.
These ratios are usually converted into "gear inches" I'll let someone else explain what it means. The 45 -18 is 66 gear inches, and the 48-16 is 79 gear inches.