Atheism is simply the position of not believing that gods exist, so by that measure, atheism is not determining what truth is or how we get there.
However, in rational skepticism (my form of atheism), we determine if a statement/belief/proposition is true by how much that statement comports to reality and can be substantiated with the data at hand.
For example, if I say "the sky is blue", all we would need to determine that is determine what is blue, then get a wavemeter and compare the colour of the sky to what blue is.
If, for example, we say "Bob stole the jewels out the safe", we would then need to do a bit more work:
Collect forensic evidence from the safe, security camera footage, geolocation data from Bob's phone, wire taps of Bob talking to his associates about hiding the jewels, etc.
The data is there, and we can build the case that Bob stole the jewels.
Skeptics are of the position that that data is not there for gods.