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The device i.e phone. tablet etc your using may have a built in data counter / monitor in it's settings to be able to see how much data has been used @bmob
Current Phone > Samsung Note 20 Ultra 5G - SM9860 Snapdragon.
Samsung One Ui 3.0 / Android 11.
The best way is through your online account, the only problem is receiving the log in security code and this will be sent as a pop up on your dashboard, when you are logged in it is worth setting up an alternative contact number to receive the code, this does not need to be a Vodafone number.
As mentioned by @BandOfBrothers you will be able to see how much data has been used in a single session from your dashboard but this is the device counting, data usage is measured on the network always worth a double check.
At least your allowance should be automatically capped and you won't be able to go over and incur extra charges without buying extra data and Vodafone should send a notification when you are close to the limit.