How do I cash a check with a Durable Power of Attorney document for my uncle?

It somewhat depends on the size of the check. For example-if it is less than $100 and needed for immediate care-you could sign as DPOA for him and probably cash it with proper ID and copy of the document. If it is a much larger check-the proper plan would probably be to set up a bank account in his name with you having DPOA over the account and expend the money for his benefit as needed.

Go to his bank, endorse the check by signing the back of the check "his name by your name, his agent" or "his attorney in fact" and bring the power of attorney with you. The bank should have no problem cashing it unless it is for a large amount, in which case you may need to deposit it first and wait for it to clear.

I agree with the other counsel. If the check is small in amount the bank will probably allow you to cash it if you produce the durable power of attorney and you endorse the check by signing the back of the check "his name by your name, his agent" or "his attorney in fact". They will ask you for your identification as well. If it is a larger check then the bank will ask you to deposit it in a bank account in his name. You will then have power of attorney on the account and be able to spend the funds on his behalf.