What if I can be claimed as a dependent by another taxpayer but do not know if I will be claimed in their return, what do I do?

[Hal_Al]

It depends on what tax attribute you are trying to claim. 

With the tax law change, effective 2018, most dependents will get the same refund whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. However, you only qualify for an education credit, if you are not a dependent.

[JohnB5677]

The simple answer is to ask them.  

• If both of you choose to claim you as a dependent, the second one to file will be rejected.
• At that point the only way to file will be to mail it in.
• The IRS will send both of you a letter noting that both claimed the same dependent.
• Based on the responses to the letter the IRS will determine who is correct.
• The alternate person will not get any tax benefit and will have to pay any additional tax.

[Hal_Al]

Whether they actually claim you or not, you have to check the box on your form 1040 that you CAN be claimed. If you do that, I don't believe anybody's e-file will be rejected. 

But, you have to tell TT whether somebody actually claimed you, if you are trying to claim an education credit (there may be other tax attributes that are likewise affected). If you're filing a simple return, it doesn't matter.

If the student actually has a tax liability, there is a provision to allow him to claim a non-refundable tuition credit. But then the parent must forgo claiming the student as a dependent, and the $500 other dependent credit.  The student must still indicate that he can be claimed as a dependent, on his return. This is worth up to $2500 (AOTC shifts to all non refundable). You cannot claim the education credit, unless you know, for a fact, that they did not claim you as a dependent. 

As @JohnB5677  said, it is best if you find out.  If you can't, you need to tell us a little more about what your tax return entails. 

[Mike9241]

doesn't matter whether they do or not. on your return you must indicate you can be claimed as a dependent.