How to find out if a website is vulnerable to SQL Injection?

As we had discussed earlier about the SQL injection, it  is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. We discussed there about the login screen bypassing, that might have been beneficial to the so called script kiddies, who approach hacking just through available scripts and codes on the web, without particular interest in the field. Anyways, you came to know, who exactly are the script kiddies.
We learnt the basics about what is SQL Injection but how can you find out if a website you are testing on, is vulnerable to SQL injection or not? Fine!! Some might be knowing, but for those who don’t know, I am going to quantify the whole process.
1. Use google dorks to find out the vulnerable sites, putting the following queries on google search engine:

inurl:index.php?id=
inurl:trainers.php?id=
inurl:buy.php?category=
inurl:article.php?ID=

Now you get a list displayed on the result page. Select one by one. Suppose we select the first result.Click on it.
2. Put  ‘ (single quote) at the extreme end of the link displayed on the address bar and press ‘enter’.Or after The "Equal to(=)" Sign
3. Now if a page opens up saying there is an SQL Error, that means the website is 110% vulnerable to SQL Injection.