When does turbo tax let me know which 1040 form will be the best choice for me??

[Carl]

There is no choice. There is only one 1040 form. That's it. The 1040-A and 1040-EZ no longer exist starting in 2018.

[Texas Roger]

There is now only one new 1040 form plus 6 new schedules (1-6)that are often needed to go with it. TurboTax will choose which schedules, if any, are needed to go with your 1040. If you need any of the new schedules or one of the old schedules that didn't change much such as Schedule A, C, D, E or F, that are not supported by the version you are using, Turbotax will recommend an upgrade.

You can look at what's included in each version to help you understand whether the version you are using will be adequate or not: https://turbotax.intuit.com/personal-taxes/online/