So how do I know that the strut needs replacing?

Drove into town today to replace the tires - just in the nick of time as old ones looked real bad on the inside, but they were over 6 years old and trailer parked in Mojave desert.

Anyway, I noticed that if I come to a stop while driving forward, the tongue remains compressed. When I release the hitch and crank it up, the tongue ever so slowly extends back to full forward. This probably means I need a new strut...

The surge brake is working well and tongue cylinder is reasonably well greased.

Of course, I parked it back at the airport and didn't think to take a look to confirm what might need replacing. It's a 2001 Cobra with square tongue.