how do i know if i need
to clean my gutters?

3 things to look for

Staining on the outside of the gutter

This is caused by the rain mixing with the debris sitting in the gutter and running over the edge over a period of time.

Washed out Mulch or Soil around the foundation

This again is caused by the rain running over the edge of the gutter and over a period of time washing out the mulch or soil around the foundation.

Discolored Pavement/Concrete or decking:

Over time even your driveway, walkway, or decking can become discolored due to a constant overflow of water from your spouts..

Most Homeowners...

Most homeowners dismiss their gutters until it's too late and wind up suffering the consequences. You should, in most cases, do this procedure 4 times per year. This doesn't necessarily mean a cleaning would be required. However, keep an eye on them. You don't know what may clump up in your spouts- tennis balls, birds' nests, a stick, roofing sediment. That is all in addition to the seasonal debris- seeds in the spring, leaves in the fall. Again, check them quarterly for proper flow during rain and expect to need a cleaning 1-2 times per year

So how do i know if my gutters are clogged?

To accomplish this without pulling out a ladder, I would walk around the outside of the house looking up at the gutter from underneath, for discoloration of the gutter. This will indicate there is water running over the lip of the gutter and down the face of the gutter. 
...then I would walk around the outside of the house again, only this time, looking down, at the ground under the gutters, for the soil or mulch to be washed out around the foundation of the house. This will indicate the water is running over the lip of the gutter wearing a hole into the ground. In the event the gutter is over
the driveway or a walkway, you should look for discoloration on the area under the gutter.