The test is not so much that your mum has dementia, more is she able to understand what she is signing, & what the consequences for her are... In that regard her Dr can do or organise cognitive tests to ascertain her 'legal capacity'
If she is deemed to not have capacity i.e. scores very low on the test, you cannot get her to sign anything after that as she is incompetent. She should see a geriatrician for this test.
If it does end up being too late you will need to go to VCAT to apply for it. VCAT will probably do their best to assign financial management to The State Trustee instead. You must be prepared to fight hard against this. And VCAT will also try to grant Guardianship to the Office of the Public Advocate.
See last year's Four Corner's expose as to why these government agencies are to be avoided. You can find that and other stories at the Australian Public Trustee & Guardian Injustices Exposed.