How do I confirm that I'm not over-paying SS Tax and Medicare Tax (even though I have income in two states)?

[SteamTrain]

Income in two or more states is immaterial......it's income from separate employer's W-2 forms that you normally have to be aware of.

The total of your own, different employer, W-2 forms, box 4 values, should not exceed $8239.80 for 2019 taxes.   That's the MAX you have to pay into SS.   On each W-2 form, box 4 should not be more than 6.2% of the box 5 value....and a W-2, from one employer, should not have a box 3 value greater than 132,900 for 2019 .

There is no max for Medicare , but box 6 on each separate W-2 (if  you have W-2 forms from different employers) should not exceed 1.45% of box 5

[VolvoGirl]

If you had more than one employer and  the total of box 4 (only box 4 not box 6) on all your 2019 W2s  for Social Security is more than $8,239.80 you get the excess back on your tax return.  And it is for each spouse separately, not combined.  

Check 1040 Schedule 3 line 11 for it.  Then Schedule 3 goes to 1040 line 18d.

For 2020 the max is $8,537.40 on $137,700 of wages.