The best and most accurate way to find out if you should file jointly or separately with your spouse is to prepare the tax return both ways. To do this in TurboTax Online you will want to create separate accounts and returns to see the most beneficial (jointly, separate spouse 1, separate spouse 2).
For a faster method, you can compare filing jointly vs. separately with TurboTax's free calculator TaxCaster. It will give you the estimated tax differences when filing either way. It’s up-to-date with the latest tax laws and can estimate your 2018 taxes as well.
See this FAQ for more details on filing separately vs jointly: https://ttlc.intuit.com/replies/3288477