Something simple like "I was thinking about getting tested for Asperger's as I think it may explain some of the problems I've had through the years. What do you think?". This will prompt further discussion and she may end up wanting to get tested.
Also, you'll probably get some screening questionnaires to fill out, so get your girlfriend to have a go at them as well.
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Diagnosed: Asperger's Syndrome (ICD-10)
Self-Diagnosed: Aphantasia
Your neurodiverse (Aspie) score: 152 of 200
Your neurotypical (non-autistic) score: 46 of 200
Listener of all things noisy, viewer of all things bloody, writer of all things sh*t.