Gass, ceramic, and plastic should be safe (if you look at plastic items and see the recycling triangle thing, there are numbers and certain ones are food safe but I don't remember which, lol). Most things food-safe are aquarium safe. Absolutely no metal and be aware of paint and small holes. Also, soaking in water to see if any color leaches or any weird smells come about is a way to test, but there isn't anything that's 100% certain, honestly. Even some decorations made for aquariums are dangerous so it's really a personal decision safety-wise.
IMHO, the safest things are live plants and silk plants with no metal wires, or terracotta pots with the holes filled as they are simply baked clay. It's probably not likely that MOST things would be an issue but there is always a risk. If you put in a new decoration and your fish starts to act lethargic or strange soon after, it should be removed, followed by a couple large water changes.
I know that sometimes buying decor not specifically made for aquariums is cheaper. It's done often but you need to use a bit of common sense and watch any new decor closely for a week or two. Also if you're really not sure what the sculpture is made out of then it's probably best not to use it, but if its really neat you could always set it next to the tank for looks, lol :)