In tests I have seen that:
sp_spaceused
won't work as expected, it showed 12GB after deleting lots of records. And the other answers regarding query sys.databases were not clear enough to me.
Searching around I found a very good explanation regarding SQL Server 2012 Express Edition 10GB Size Limit on Ramons weblog [EDIT2018 updated link]
SELECT
[name] AS [Filename],
[size]/128.0 AS [Filesize],
CAST(FILEPROPERTY([name],'SpaceUsed') AS int)/128.0 AS [UsedSpaceInMB],
[size]/128.0 - CAST(FILEPROPERTY([name],'SpaceUsed') AS int)/128.0 AS [AvailableSpaceInMB],
[physical_name] AS [Path]
FROM sys.database_files
"... space includes the transaction log and it also includes all unused space within these files. .... SQL Server Express will start complaining when it cannot reserve any more space for the datafile."
So checking
CAST(FILEPROPERTY([name],'SpaceUsed') AS int)/128.0 AS [UsedSpaceInMB]
seems to be the best option.
In combination with EF in c# my request to the DB looks like
string sqlSelect = "SELECT CAST(FILEPROPERTY([name],'SpaceUsed') AS int)/128.0 AS [UsedSpaceInMB] FROM sys.database_files";
var dbResult = dbInstance.Database.SqlQuery<Decimal>(sqlSelect).FirstOrDefault();
double spaceUsedInGb = Convert.ToDouble(dbResult)/1024;
express
version to build the project. – Disruption