A word for someone who notices things very keenly
I think the set phrase to be onto something expresses the acute sense of noticing something, as only the word awareness can suggest:
in or into a state of awareness about
(Merriam-Waster)
Another good word for it, perhaps even better, is the phrasal verb to sniff something out:
to discover someone or something, usually only after a special effort
Perspicacious
According to Wikitionary
(figuratively) Of acute discernment; having keen insight; mentally perceptive.
Your mother has a perspicacious nature. You can modify your sentence to use this word.
In simpler words:
notice changes quickly
The word that comes to mind for me is spot:
... unfortunately she spots your attempt and says ...
In this context, spot means to notice something:
spot
VERB
[WITH OBJECT]
- See, notice, or recognize (someone or something) that is difficult to detect or that one is searching for.
‘Andrew spotted the advert in the paper’
‘the men were spotted by police’(Oxford Dictionaries)
Although, if she's anything like my daughter, her attempts to change the subject will be obvious... 8^)