When I am sorting out the wheelweights, the obvious giveaways are "Zn" (zinc) or "Fe" (steel) marked on the weights. The lead stick-on type can be easily bent and are nearly pure lead. The Zn and Fe ones take a lot of effort to bend at all. For the clip on type weights, I keep a pair of pliers or wire cutters handy. Clamping down on lead weights will distort the weights with very little effort. The other ones are definitely much harder.
Clip on wheelweights make great bullet alloy as is. For handgun velocities (below about 1300fps), just cast them and allow them to air cool. For rifle velocities (above 1300fps), you want to either gas check them or "water drop" them to get the hardness much higher and not lead your barrel so much. You could also heat treat them in the oven for a while if the boss ain't going to be around for a few hours.
Be careful when melting the wheelweights. If you happen to contaminate your alloy with zinc, you are pretty well S.O.L. It is next to impossible to get good mold fillout with contaminated alloy and is a pain to get cleaned out of your pots. A good indicator of zinc contamination is a layer of crud that looks kind of like oatmeal floating on your melt. Contaminated alloy can be used, but to have any success, you have to have temps way high and then you run much greater risk of lead poisoning gases as well. Always make sure you have a very well ventilated location whenever you are melting your lead.
You can avoid the zinc contamination by keeping your temps down below about 750 or so. Most guys even keep them below 700 and can skim them off the top with the rest of the garbage.
Good luck and happy shooting.