I am very new to this site (great site, btw), and have looked around for a similar Q, but found none. Much online talk about sqrt(2) being irrational. A quick Excel spreadsheet inquiry shows a LOT of square roots that look like they're irrational. It seems like only perfect squares and multiples of perfect squares (4*49) have obviously rational square roots (they're integers). Is there a way to tell the difference between a true irrational number and a huge rational number? (1/499 repeats for 500 digits) And finally, of all those irrational-looking decimal numbers, which is the first one (not a perfect square) to be deceptively rational?
I have my money on 18^.5, but I do not know how to tell what is what.

Thanks in advance
-Jim Huddle