Is there any tool where I can enter a domain name and find out what adwords keywords the site is using?
Most of the site is a paid service (ranging from $100 - $900?)
Try Spyfu.
iSpionage and keywordspy are alternatives.
There is actually a script you can use that doesn’t require a monthly fee but I forgot the name of it.
I wouldn’t mind signing up to one of them but none of these sites accept paypal as a payment method, I’m weary about entering my details on other sites.
Any recommendations using this tool? Is this a paid tool or free? :x
Costs $900 annually… I only want a tool to investigate one thing 
never heard about adwords tools …
please tell benefits if using adword tools ??
Thanks for answers …
Well it depends on what degree of detail and comprehensiveness you want. The non-paid check gives you quite a bit…
you can use iwebtool thats really usefull for you
semrush.com is good for this purpose
Wow! That was too much. 900$ can be one’s salary for a month. Any other cheaper tool? 
well the best free service is from adword itself…
https://adwords.google.com/select/KeywordToolExternal
remaining all will cost you and will not be much effective …