Ah,,, something I know about, lol,, I could go into all the technicalities of Standing Wave Ratio (SWR), but Roc is right,, new antennas are set pretty well from the factory and you don't really have to worry about em. Besides, you'd have to seriously alter the length of the antenna before it would be off enough to burn out your finals in the radio.
If you're really concerned about it, your transmit/receive range seems to be severely hampered or you are dealing with an old antenna, I'd stop in a Radio Shack and pick up a cheap VHF SWR meter for about 10 bucks.
Adjust antenna for the lowest SWR possible (1:1).
In a nutshell, SWR is the ratio between the amount of power leaving your radio and the amount of power that is actually coming off the antenna. The length of the antenna is critical to maintaining a balanced resistance for the frequency you're transmitting on. If your antenna is not the right length for the frequency you're using, the resistance will be too high and it will heat up the amplifier in your radio. An SWR reading higher than 1:4 would be cause for concern, and the whip on most antennas can be adjusted in and out of the base to fine tune it. The optimum SWR reading is 1:1, meaning that you're actually getting out of the antenna the same amount of power that you're putting into it. Length of the coaxial cable itself actually has very little effect on SWR, as long as it is well shielded and the connectors are in good shape.