Let's assume fairly established groups, not "lone guy with his mech". Do they have a skywatch of any sort? How could they go about detecting that group of dropships burning in and bringing an RCT full of dinner guests?
IIRC,
cray has always maintained that a decent backyard telescope and a star-chart to compare the 'current' picture to the 'proper' background will do the job pretty well on a budget. If you're looking for something a little more up-scale, a full-sized astronomer's observatory is also 'good enough', but doing that in a ground-station requires the sort of infrastructure investment capital most pirates would rather plough into (stealing) more 'Mech parts.
A couple of visual-recon satellites (even jerry-rigs cobbled together in a workshop, and
someone in the band would have to be a pretty decent 'tronicist to keep all that killware working) would be a decent substitute; they're not going to have Hubble-grade resolution, but they only have to be able to see the commonly-used jump-points and have enough communication-bandwidth to report any anomalies to a ground-station (which could probably be part of a planetary traffic-control station). IIRC, TRO:VA has a couple of satellite designs that would probably be overkill for the job but give you a good yardstick for the sort of costs and capabilities it would entail. Stick one or two of those at each of the main jump-points and set up a network of several more around the home-planet (or moon, or asteroid) watching the whole sky, and you're away laughing.
And of course, if this pirate-group actually expects to get anywhere to
be pirates, they'll have their own JumpShips at or near the main jump-point(s), ready to report any new arrivals at the same jump-point(s). Assuming the duty watch is awake enough to radio a spotting report to the planetary base before they get swarmed by the guests' fighters, that's about all the 'skywatch' they'd need. ;)