How Can I Find The Sale Tax Rate From The Subtotal Or Total Amount From An Invoice?

If you don't know the rate of sales tax and have the sales tax amount you can use this simple method for finding sales tax. Taking your numbers:
 
Sales amount = 714.91
Sales tax amount 55.99
The formula is
X% of Sales amount = sales tax
x/100x714.91 = 55.99
x=55.99x 100/714.91
= 7.83 percent is the answer.