You should check at least annually to remove any debris that could clog the valve and to make sure all moving parts have free movement. Most valves have an easily removed cover, to allow quick cleaning. But be careful. Some experts recommend running some hot soapy water down your sink first to make sure the system is fairly clean. Always wear rubber gloves and use a long handled brush to scrub around and under the flap. Most backwater valves will have manufacturer’s recommendations regarding maintenance.
You’ll also want to look at the O-ring around the lid to make sure it’s in good condition. If not, it should be replaced to ensure a proper seal. Also, check the floats at each side of the flap, and replace if necessary. If these are worn, they will lose their ability to float, and won’t be able to lift the flap when needed.
To keep everything flowing smoothly, you may want to consider NOT flushing such things as “flushable wipes” or diaper liners. By disposing of these things in the garbage, rather than in the toilet, you will be saving your system from performing extra work, and will hopefully be preventing the system from getting clogged.
If you’re unsure about the condition of the valve, or are not comfortable attempting it yourself, you can call a plumber to do an inspection.