Usually, you will use special purpose scanners (designed for web application testing). There are many web app scanners that do a good job at detecting sql injection. However, no tool is perfect and its is recommended to not rely on any one tool. If you have 3 web app scanners, run them all then compare your results. Also keep in mind that it is possible that your scanner wont find every flaw in the application, so it is absolutely mandatory to have some knowledge of performing these tests yourself manually, to verify scanner results.
One tool that i find useful in detecting sql injection flaws is proxystrike. Proxystrike is a proxy/scanner that looks for sql and xss vulnerabilities. From the proxystrike documentation;
The process is very simple, ProxyStrike runs like a passive proxy listening on port 8008 by default, so you have to browse the desired web site setting your browser to use ProxyStrike as a proxy, and ProxyStrike will analyze all the paremeters in background mode. For the user it is a passive proxy because you won’t see any difference in the behaviour of the application, but in the background it is very active.
In the video below, i demonstrate how proxystrike makes it easy to detect sql injection vulnerabilities. I use DVWA (damn vulnerable web app) in the demo. To verify the vulnerability found by proxystrike, i used sqlmap to test and exploit the flaw. The end result was a dump of all the databases on the database server.