is there a way to check whether Magento store is secured or not just by entering the URL of the store
There are a number of tools that can be used, however security is very much a complex matter, you can wrap it in many layers of security measures but if the password to the server is still "password" you have a pretty major security risk going on there.
That tool will do a quick audit and see if any sensitive areas are easily reachable (admin for example). It will try to detect modules that are installed as well (not always successful from my experience of playing with it).
https://shoplift.byte.nl/ as mentioned above is another useful tool, although so far I've seen a few false positives.
If you are concerned about security the best advice would be to contract a PCI-DSS Compliance Auditor to review the full site and architecture, they will give you a very detailed (we recently had one done and it was in the 40+ pages worth of report) that details every aspect that they can potentially review. It may be overkill for your needs - particularly if your not actually handling sensitive information in the context of PCI Compliance but if you are compliant with it then you should be able to be comfortable in saying your site is secure.
If you are looking to see if you website is secure from the known recent vulnerability(the "shoplift" bug which is solved by the security patches) you can use https://shoplift.byte.nl/ to check for that you just need to put your stores URLS. For general security if you register you website with Google webmaster they apparently inform you if you they noticed a security issue. Another option is using website scanners(i have never used them so don't know how effective they are). You can find a lot through a search engine