You can view active schema locks by right clicking on the database and selecting Administer Geodatabase. In the popup, select the Locks tab and you can filter by Lock Type = Schema. This will tell you what features are actively locked. Depending on how the SQL rights are assigned in your organization, you may be able to see discrete usernames in the Lock Owner column. If you don’t see usernames there, you can check out the Connections tab, where you can see who is connected to the database to narrow down your search.
From my experience, schema updates are best deployed outside of working hours or during planned maintenance windows when only you are connected to the database. Even if users aren’t editing the exact feature class you would like to update, the dataset as a whole is locked if there are active edit sessions connected to any feature within that dataset. (Assuming you are using datasets) Tables, I believe, are fair game if no one has a schema lock on it.