How can I identify lead-free solder if it's unpackaged?
Try soldering a 0.5mm pitch component with it. If you get frustrated and want to throw it across the room, it's lead-free solder.
Use one of these lead-check swabs:
[edit: link updated]
http://leadcheck.com/
https://web.archive.org/web/20160203163608/http://leadcheck.com/
Here is a new link to the same product. 3M Lead Check Swabs
Keen or desperate? - measure it's specific gravity.
Compare with published or calculated data.
Take a large enough sample and weigh it = M1.
Suspend in water and weigh again = M2.
SG = M1 / (M1-M2)
or
Weigh = M1.
Then measure volume by water displaced when inserted into a just full container.
Water mass = M2.
SG as above.
Lead solder makes "shiny" joints when properly used.
Lead free solder makes more matte appearance joints.
Melting points vary.
Lead chemistry, various.
Based on Leon Heller's link
Various lead test kits are available.
These are usually intended for testing for lead paint but should be very effective with solder.
You can do DIY test solutions using Sodium Sulphide solution. Here is an excellent article explaining Sodium Sulphide testing for lead. This test can be used on hands etc to check for lead contamination and to show how effective cleaning proceures are.
Excellent discussion -Science Fair project starter on lead testing