How can I check if my log is truncated?
In simple recovery mode the log gets truncated automatically when checkpoint is issued.  Now when you run DBCC SQLPERF(LOGSPACE) and your log is more than 70% full then you can assume that your log is not getting truncated because a checkpoint is supposed to be issued when the log grows to 70% in simple recovery.

If you suspect that your log is not getting truncated in simple recovery you can use sys.databases, log_reuse_wait_desc columns to find out why the last log truncation did not work.

If you wanna dig a bit deeper, you can use dbcc loginfo and check the status column to see which virtual log records are active and in that way figure out if its getting truncated or not