To the OP: btw, was there also a warhead interceptor on the planet?
I use a rough procedure to identify most of the stealth guardpost before I assault a system...
1) With scouting information mouse over the cloaked ships count (circle on the right-hand tab). It will mention a stealth guardpost if there is one. Also, if you see things like Missile Frigates cloaked - that's also pretty much 99% chance of a stealth guardpost existing (AI has only one, possibly two other options presently of super-cloaking).
2) Check seemingly empty resource spots, followed by Fortresses and other items without an accompanying guardpost.
3) Attack system and wait for ships to come out of the blue. That's the place, factoring in the cloaking range (a few thousand units?). Send some stuff to camp over the spotand it will reveal itself by firing. Their firing range is somewhat small, so one might just miss it with a few raid-starships.
Addendum: If not found yet at the end of the assault (say, if I forgot and there's no ships left around the stealth post to give it away), run several decloakers through the system. The scout starships have better range nowadays and can also suffice... (use mark-IIs or better and formation move like a comb)