1. The mitzvah of Matanos L’Evyonim on Purim is to give at least one gift to two different poor people on Purim day. (S.A. 694:1)
2. There is a great discussion amongst the poskim as to the proper amount to give to each poor person. The Ritva writes that even a perutah (i.e., a minuscule amount of money) may be given to each of the two poor people in order to fulfill this mitzvah. However, others maintain that practically one should give enough money with which the poor person can purchase for himself a decent meal. (see Yismach Yisroel page 121 and Halichos Olam vol. 1 page 233) I have heard from Rabbanim that the amount of money that a poor person would need to purchase a bagel, coffee, and some other small side dish or two slices of pizza and a can of soda, definitely suffices for the minimum amount required to be given to each of the two poor people. For practical halacha, a rav should be consulted.
3. The poskim maintain that giving a check to a poor person fulfills the mitzvah of Matanos L’evyonim. (Rivevos Efraim 5:455:2 and Halichos Shlomo Purim page 342)
4. Harav Shlomo Zalman Auerbach zt”l explains that one can use a check even if the bank is not open on Purim. (Halichos Shlomo ibid.) This is also the view of Harav Yisroel Belsky zt”l (Piskei Harav Belsky page 124)
5. Harav Belsky zt”l (Piskei Harav Belsky ibid.) explains that the custom is not to give a post-dated check for the mitzvah of Matanos L’Evyonim. He then adds that some poskim opine that one should not give a check if he does not have sufficient funds to cover the check. However, if one will accrue the funds by the time the poor person cashes the check then it is permitted.