I really don’t remember if I gave maaser on a couple checks I received a couple months ago, what should I do?
It depends on what you said when you started giving maaser.
If it was done in a way that giving maaser is a neder, you should be matir the neder, you have plausible reason that it shouldn’t be a neder. Then you don’t have to give maaser for it. (After being matir neder say that any money that you are going to give from now on for maaser should be bli neder).
If you said that your giving maaser should be bli neder then you also don’t have to give for them. In any case although you don’t have to give the maaser it would be a commendable thing to do.
As a side point it is always a good idea that when a person starts giving maaser, for him/her to say that it is all bli neder.
Shevet Hakehosi 6- 396, B’orach Tzedakah 2-29.