I'd like to purchase a tarp as some point, both to put up over a small camp at some point, and to use over a hammock.
When I've camped with my uncle he's used a folded over builders tarp, which has worked so far but I have seen other that have leaked, either 'out of the box' or due to fire damage (hot sparks from a fire causing tiny holes).
What are the characteristics to look out for in a quality tarp, and how can I spot them before I buy them?
Most tarps have a coating applied to them. DWR should be avoided, it's only water resistant and will eventually drip. Silicone impregnated (aka silnylon or sil-nylon) is lighter than polyurethane-coated, and is supposed to be equally waterproof, but some people say silnylon can mist during heavy rains but it's more likely from condensation than permeability. A tarp that is polyurethane coated will usually have a flame retardant element added to it. Some tarps are coated with both in order to meet flame retardant standards. Polyurethane coatings are always on the underside, and may peel with time. Silnylon strengthens the fabric, while polyurethane weakens the fabric over time.
TLDR; If you have money and weight is of importance, go for silnylon, otherwise go for polyurethane.
First, you have to figure out the dimensions you're looking for. Too big is uselessly bulky to carry, but too small can prevent it from being useful. This will depend on the size of your tent/hammock or whatever else you're trying to protect.
Apart from that, things you can look for are:
The big difference in price / quality is the weight. A builders tarp that would normally be transported on a truck does not need to be as light as one that you would carry on your back.
There are also some that have design details, like being able to combine 2 to make a tent.
There are a few things to think about:
Fabric. Selection of fabric will affect weight and durability. Whatever you get, make sure it's UV resistant, and you should probably consider something with rip-stop. For me, the ultrasil nylon is a bit light (I doubt it would handle much of a tree branch falling on it). At the same time, a durable canvas tarp would be relatively heavy. Mine is a 210T rip-stop nylon -- not the lightest, but it's plenty durable.
Dimensions. Too small is useless, too large is extra weight. I use a 10x10 foot tarp, which is quite spacious. In fair weather, it is plenty large (possibly too large!) for my hammock. In the rain, I have plenty of room to relax and cook beneath before stringing up my hammock. I'd consider an 8x8 an absolute minimum.
Weight. Obviously, this is a concern if you're thinking about a tarp, but it's a trade-off between fabric and size. You'll have to make your own call on what's acceptable.
Construction. This will not add much to weight, but may add a great deal to both overall quality and price. Check the seams on the edges and how the grommets/tie-outs are attached - a brisk wind could easily tear these. Seams should be seam-sealed. Compare a few vendors product, and you'll begin to learn what works and what does not. Find someone who knows something about sewing and have them look (my wife sews professionally).
Positioning of the tie-outs or grommets. This will affect how your tarp can be set up. Because of the positioning of attachment points, some tarps must be set up either square or on a diagonal. If this is important to you, be sure you check first!! Ideally, you should have loops which allow the ridge line to be run above the peak of the tarp rather than pitching the tarp over the ridge line. Allowing the ridge line beneath the tarp will allow water to run down the lines and drip beneath. Further, there should be attachment points in all four corners as well as at least one point in the middle of each edge.
I'd suggest going on YouTube and looking at some of the tarp videos. Many are quite thorough, including the various knots you will need to learn and methods of rigging and positioning the tarp for various use cases. I learned a lot when I was first shopping for tarps.
Finally, I would not buy a tarp online without having seen it in person. It's difficult to judge the quality of construction or durability of the material without actually having it in your hand.
