Never enough Q's
You can check it any time but try to do so at around the same time each day. To check, insert 2 fingers and just see how it feels... is it firm or soft? Low (easy to reach) or high (not so easy), does the tip feel open or closed?
I used to find it difficult to tell the changes between open and closed so I would just mark if it was high or low and firm or soft.
this link is quite graphic but may help you. I found it useful when I first started checking.