How can I see the 1040 form before I file?
You cannot see all of the forms and schedules before you file when you use online. You can preview your 1040 and Schedules 1, 2 and 3.
PREVIEW 1040
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/preview-turbotax-online-return-filing/L77WCkvnu_US_en_US?uid=m681fkhr
If you pay your fees then you can print or see the rest of the forms and schedules.
https://ttlc.intuit.com/community/printing/help/can-i-print-a-copy-of-my-turbotax-online-return-before-i-file-it/00/26244
with the desktop versions you pay for the app but you have free access to view any and all forms.