Won't garentee anything, and throw in the China 'quailty' issues and who knows....
But a 40 chain tends to be silver color that rusts quickly to rust with no marks at all.
Gr 70 tends to be the gold color, and doesn't rust nearly as quickly. There may be a '7' or '70' stamped on the links or hooks; but of course hooks coulda been swapped out if it's an old chain.
Just found this on the web:
Chain Grades
Grade 30 Proof Coil Chain
A general purpose chain of standard commercial quality. Made from low carbon steel, this chain is frequently used for fabricating tow chains, logging chains and, when appropriate, tie down or binding chains. The manufacturer's symbol and grade marking: 3, 30 or 300 is placed on the chain links every foot or so. Not to be used for overhead lifting.
Grade 40 High Test Chain
Is a higher strength chain used for years in the trucking industry for tie downs that meet DOT specifications. Made from a higher carbon steel, its strength surpasses proof coil working load limits, size for size, by a factor of 2 to 1. The manufacturer's symbol and grade marking: 4, 40, 43 or 400 is placed on the chain links every foot or so. Not to be used for overhead lifting.
Grade 70 Transport Chain
As its name implies Transport Chain is used primarily to tie down loads on over-the-road equipment. Made from heat-treated carbon steel it has about 25% more strength than high test chains. The manufacturer's symbol and grade marking: 7, 70 or 700 is placed on the chain links every foot or so. Not to be used for overhead lifting.
Grade 80 Alloy Chain
Designed to stretch a minimum of 15% before it breaks, alloy chain is the only grade of chain approved by OSHA and other agencies for overhead lifting. Its alloy, heat-treated steel makes it ideal for making lifting slings and heavy duty tow chains. The manufacturer's symbol and grade marking: 8, 80 or 800 is placed on the chain links every foot or so. Suitable for overhead lifting.
Grade 100 Alloy Chain
The most recent alloy chain provides about 25% higher working load limits than Grade 80 alloy chain with some limitations reported in lower ambient temperatures and availability of fittings. The manufacturer's symbol and grade marking: 10 or 100 is placed on the chain links every foot or so. Suitable for overhead lifting.