There are several factors that contribute to whether or not you will enjoy your paddling experience. There is no need to learn them by trial and error the way many people do. If you have access to the internet, you can check conditions before you decide whether the trip is on.
Tides - It is critical to know how the tide will impact your trip. There are up to two high tides, and two low tides every day. When the tide is going out, the current will take you out. This is called the ebb. On the way in, this is called the flood. In between tides, the water is still, and this is called slack tide. The impacts of tides are very localized, so you can get very strong currents. These can either work for you or against you depending on your planning, or lack of planning. For beginners, it is best to go out and in again during the slack tide. For experts going out on longer paddles, you'll want to take advantage of the tides to take you further on your way out, and bring you back in. Use the internet to find tide tables for your area. I recommend saltwatertides.com. The closer you can get to your exact put-in spot, the better. (More about tides)
Temperature - Dress for the water temperature, not the air temperature. It is important to know how long you can survive (click here then scroll down for the table) in the water before hypothermia sets in. If the water is below 50 degrees F, then you will want to be sure to wear a wetsuit. Make sure you have practiced re-rentry before you go out in water that is too cold for you to survive in for more than an hour.
Wind - Very much like the tide, the wind can either work for you or against you, and it is more unpredictable than the tides. Wind patterns often repeat a daily pattern, so your best bet is to be observant of how the wind pattern varies from day to day. Usually, we prefer to kayak when there is minimal wind, which I would define as 10 MPH or less. Above 10 MPH you will be fighting the wind in one direction or the other. For fun, you might carry an umbrella or a kite to take advantage of the wind when it is blowing your way.
Sunset - Always be aware of how much daylight you have, and be prepared to be out after dark if you're pushing it. That means having the appropriate signaling gear (lights and a whistle) on board. Also, make sure you have enough warm clothing in case you are out after the sun goes down and it gets colder, and the wind picks up.
Boat Traffic - If you're paddling in an area where scheduled boat traffic can be taken into account, then be sure to know those schedules. Where I often put in on the Napa River, we have ferry service that is easily predictable. You don't want to cross the path of the ferry if you know it is going to coming by. Always stay out of the way of boat traffic.