Another way to know if the strainer clogs and flow is reduced to the engine is to install an outboard water pressure gage. Be sure to install it between the strainer and the engine, not between the pump and the strainer. Reason being is that if it's between the pump and the strainer and it clogs, it will continue to read pressure even though there's no flow to the engine. With the T fitting located between the strainer and engine, the instant it clogs, your PSI reading will drop. Running 1/2" lines, mine reads around 10 PSI at cruising speed, and around 15 PSI at full throttle.
It's important to note where your normal operating PSI is, so you can detect when something isn't right. If you see a drop in PSI, you've got a clog in the strainer, or somewhere between there and the pump. But if you ever see a RISE in the pressure, this is an indication of a collapsed or pinched cooling hose, or a restriction in the engine, such as sand or shells blocking cooling passages (which hopefully, the strainer will prevent)
On a final note, since I run a Jabsco strainer on my boat as well, I can tell you that the fine mesh screen that comes with the 1/2" model is entirely too small for jet boating, it will quickly load with the smallest of debris that can easily pass through the engine without problems.
The larger mesh screen that comes with the 3/4" model is more appropriate, as it is slightly more coarse, and will allow fine sediment to pass through, but will stop pieces of shells or tiny bits of gravel and other such debris that could jam up in the cooling passages. As you can see in the pic, I have mine mounted right next to my seat, on the front side of the rear deck. This way, I can actually see the water going to my engine at a glance, and I know if I need to stop the boat and clean the screen. And in the left corner of the pic is my flush fitting, so everything is right where I need it.