Click HERE for one of the standard fitness tests that @nebur86 mentions.
Ride it entirely in Slope mode exceeding targets if able and make sure to ride to failure on last interval.
Since you’ve manually set a starting TP, you may not get a Breakthrough (BT) performing this test, but you will get a good idea if your numbers are in the right ballpark.
Here’s the other fitness test workouts in the Library –
With Xert any maximal efforts will do to keep your signature dialed in over time.
Rather than relying on formal tests, those efforts might occur when you go-for-broke at the crest of a hill, during a lively group ride or ZRL event, sprint up a short hill, or a DIY fitness test using Magic Buckets (XMB) on your Garmin or EBC Android.
You could also do this while free riding on Rouvy/Zwift/etc. I.e. replicate the standard test – two short sprints, full rest, then a series of escalating intervals >TP until failure.
Fitness test workouts don’t show up on recommended lists, but you will see a warning under your name when Xert considers your signature as stale. I.e. time to validate it however which way you prefer.
In between BT’s decay is in play meaning your signature will start to decline if your current TL isn’t enough to maintain it.
OTOH during a progression your TP will rise as TL rises. No BT required. 
Reference –
- Fitness Breakthroughs – Xert
- BT types: Pull-down, Push-over, Multiple and Near: Breaking Through the Xert Way! – Xert