One thing to look (not just for this, but for with all aspects of aviation) is evidence of a Safety Culture. A skydiving operation is no different.
Skybrary defines Safety Culture as follows:
Safety Culture is the way safety is perceived, valued and prioritised in an organisation. It reflects the real commitment to safety at all levels in the organisation.
It should be clear to you as you interact with the company what their commitment to safety is. Do they have materials on the walls of their place of business with safety reminders? When you talk to them, do they appear to take safety seriously or does it seem to be an afterthought? Do they take time to address your concerns or do they brush them off?
It really comes down to professionalism. Do they wear uniforms, keep schedules, offer clear communication? It's not that these things make it safer, but safe operators tend to do these things.
If there is a unified message of safety that is repeated by everyone you contact, from the person checking you in, to the parachute rigger to the pilot, it's likely that they have a strong safety culture.
Safety Culture can have a direct impact on safe performance. If someone believes that safety is not really important, even temporarily, then workarounds, cutting corners, or making unsafe decisions or judgements will be the result, especially when there is a small perceived risk rather than an obvious danger.
You ask about preflight checks that you ensure a pilot does prior to take-off. In an environment like a skydiving operation, you might not see things like the pilot using a checklist because the pilot typically flies the same flight over and over again, many times in the same day, and likely uses a "flow" and doesn't verbalize the checks they are doing. They will likely look "relaxed" to you because they are doing this multiple times per day, but you can see, do they do things without rushing, make smooth adjustments? Again, it's not that this makes it safer, but safe operators tend to do these things.
At the end of the day though, like the other answer says, you have two options: trust the pilot and aircraft are competent and able to take the flight, or don't take the flight.
Occasionally when people look at me like I'm crazy for flying, I say "well I don't want to die, either, so I'm going to do everything in my power to minimize the risk of that happening on this flight." I'm sure the pilots of jump planes feel the same way.