How to check the postdoc opportunities in a specific lab even if there is no position-call from them?
Yes. This is how it works.
Once you find a lab, find a professor who you would like to work.
And then, 


Read his publications
Read them again to look for potential areas where you find yourself as a useful asset to that lab, like methods or instruments you are familiar with and used by the professor too.
Email the professor asking for available positions. Send in your email your CV and statement of purpose.
If you could add a document where you could be an essential asset by proposing additional ideas related to the professors published articles, it might add value if you don't sound arrogant.
While involving in such activities, if you find a wonderful result try to publish it and cite the professor's work on which you are trying to develop. This will make your application be outstanding and successful.


The right time to do so is when you are in the middle of the year before your last year of Ph.D. Also, you should let the professor know that you are writing him a year ahead so as to plan accordingly.