How do I know if I chose to hold some of my tax refund to apply to next year? I think I did accidentally because my return was a lot lower than previous years.

Check 1040 line 21.  But there could be several reasons why your refund is less.  It could be for state.  Federal and state refunds come separately at different times.  Turbo Tax just shows you the total combined at the end.

Or the IRS changed your return.  They will mail you a letter explaining why.  Or you were having the Turbo Tax fees deducted from your refund.  There is an extra 39.99 Refund Processing Service charge (44.99 in California) for that.

Or do you mean your refund on line 20a is less?  

For one thing, they reduced the withholding tables so you got more back in your paychecks each month.