Is rent tax deductible? I'm currently paying rent on one of my parents two and a half family units and thought maybe I should start paying by check so I can have proof when I file my taxes. Thanks everyone.

No - rent is not deductible, at least not from your federal taxes. Some states provide a tax credit on their income tax for renters, but it depends where you live.

I live in Boston, MA. How do I check if my state provides a tax credit for renters?

For MA you get to deduct half of your rent paid, up to a max of $3K. Look at the instruction for line 14 of the MA income tax form 1:

http://www.mass.gov/Ador/docs/dor/Forms/IncTax07/f1_nrpypdfs/1_instr.pdf

The deduction must be for rent you paid to a landlord
for the rental or lease of your principal residence
in Massachusetts.
If two or more persons jointly rent a unit, each occupant
using it as his/her principal residence is entitled
to a deduction based on the amount of rent
that each person paid.
If the rent is paid by a third party (such as a parent)
who maintains a principal residence elsewhere, no
50% rental deduction is allowed for either party.
A principal residence does not include any residence
for vacation, an apartment for a person on
a temporary assignment or a student or faculty
member who has a principal residence elsewhere.
It also does not include any apartment or house
in Massachusetts of a nonresident who has a
legal residence in another state or country.
Payment for occupying a hotel, motel or rooming
house is not considered rent unless a rental agreement
exists. All separately stated charges such as
utilities, furnishings or parking cannot be included
in rent for purposes of this deduction. Also, rent
does not include any advance payments (such as
security deposit, last month's rent, etc.) until actually
applied as rent.

You should save your lease agreement and either pay bycheck or ask your parents (the landlords) for receipts so you can document the rent payment.

Thank you for the advise