Puzzler's Edge

Given an arbitrary rational number in decimal form, how can you quickly determine the denominator of that number? (quickly as in something that does not on really large numbers or factorization, and preferably something that does not use iteration or recursion.)

Bonus points (and the problem I'm actually trying to solve): How can you make a graphing calculator (desmos.com in particular) calculate the LCD of two fractions when the LCM function is only defined on the integers?


_{yes i know this is a simple problem but i'm bad at number theory. this counts as number theory, right?}