My only advice is to have her do it while on her knees on a surface where you can clearly see liquid. (such as the bathroom floor) One problem for me was that sometimes I'd "feel" like I've squirted but I couldn't confirm it because I am a low-volume, low pressure squirter and fluids just mix together in certain positions. If she does squirt, It'll drop down right between her knees and is clearly a different texture and color from natural lube.
A mental exercise I found really helpful when I was learning was this: masturbate with a decently full bladder in a dry bathtub/shower and when you're about to cum, try to release your bladder and pee. Nothing should come out because it's rare that someone can pee when aroused. This helps the stigma that squirting is just pee, and also helps to differentiate the sensations of squirting and peeing.
Also, I've noticed that I only ever squirt if I've had an orgasm not too long ago. Hope that helps.