You can't explain your question well, so its better to handle it as two separate questions.
- How can i secure my computer from Zeus Trojan?
- How can i know if my developed web pages are infected?
 As just said make use of Anti-Virus/Malware software, update it regularly and schedule at least weekly scans for each computer.
Make sure that all of your systems software and OS are updated regularly.
Use safe browsing tools/browser extension like NoScript, Adblock, WOT and read more by yourself about what these tools can do, this will also increase your cognition about web security issues.
 Many online services are available that can scan a site in search of malicious code just providing the site link (URL)
and so on...
If you are good enought, you can develop by yourself a tool that automates the process of scan using specific APIs provided by Google or Sucuri for example.