Pre-trip inspections aren't just for semi trucks—horse trailers should also be inspected before every trip on the highway.
I find a rubber mallet an essential tool for pre-trip inspections because you can bang it on every part of your trailer, including tires to see if they're flat and find out if there's anything loose that needs attention before you haul your horse back there. You can hammer down on the flooring, all welds, bolts, nuts... virtually everything! The reason I suggest a rubber mallet is so you don't scratch your paint job, and if for some reason a bolt needs to be replaced, you won't flatten out the end with a rubber mallet as you would with a metal hammer.
A friend recently found a bunch of bolts that had sheered off and when she looked into the issue deeper, there were more also. Bolts rust and over time they can weld to the bolt. When you attempt to loosen them, they'll snap right off in "most" cases. In some cases, the bolt has to be drilled out and then it can be replaced.
In trailers that have floor boards, inspect all the channels that hold those boards in. If they're rusted through, your next trip "should" be to a trailer repair place, or any welding shop. They can remove the old channels and replace them with new stock. If you ask, they'll inspect the rest of your trailer while they're at it and bring any other objectionable things to your attention.
Aluminum trailers are not immune to issues. Checking rivets, channels and everything that moves and isn't suppose to move will save you a lot of headaches later on. If you have rubber mats down, pull them out at least once a week and look for pits in the flooring. Those pits mean that urine is eating through the protective coating and can cause holes to form thus leaving those spots nearby weaker and flexible.
Check all your doors, hinges, locks, windows. Have a look see at your trailer axles, brakes, shoes/pads, springs if you can see them. Maybe rotate your tires every couple of weeks just so you can see what's going on in there and inspect your tires while you're at it. Several ladies, in a group I'm in, have had tire blowouts in this past week. Thankfully none of them were hauling horses at the time, and there weren't any major wrecks.
I find that most folks do a quick trip around, glance at their tires and maybe check to see if all of their lights are working. Take the time to make sure your lights are working! Tail, blinker, break, and highway (those lights along the side of your trailer). If you've got lights that aren't working, check fuses first, then check the bulbs, plug in and finally wires. If a light isn't working, chances are it's a bulb that's gone out or a fuse that's blown. Those 2 are the cheapest fixes and easy enough for you to change on your own.
Give your hook up a tug and a beating with your mallet. If you hook with a bumper pull, check the hitch and receiver very good. Look for cracked and broken welds. Check the ball. Is it still round or is it really pitted and egg or oval shaped? How are your chains? If you've got 1 chain, plan on having another one installed on your trailer. All horse trailers (and all other trailers) are by law supposed to have 2 safety chains, not just one. If you're not sure of the proper way to hook them up, which is crossed and attached to the frame of your truck, not just the bed then look it up. Google is a great tool for knowledge! There are all kinds of videos on proper chain hook up and placement.
Does your trailer have a breakaway cable? Check it! It's cheaper to replace the battery for it by checking if it works than to find out it doesn't and your trailer emergency breaks don't come on or work properly.
If you haven't spent any time doing a pre-trip inspection on your trailer, I suggest doing so. This gets you familiar with what things look like, and you'll spot something quicker that isn't right so you can get it fixed before you haul your beloved horses or other animals. Sure, you can do a quick trip around your trailer.... however your animals are relying on you to haul them safely. They didn't ask to get into that box, but they'll do it because YOU asked them to, so take care of them and consider everyone else's safety too.