You can use system stored procedure sp_msloginmappings to list out the SQL logins and database user mappings.
Syntax
sp_msloginmappings @Loginname , @Flags
@Loginname – Optional argument, in case if you not specify the Login name procedure will return the result for all the SQL Server logins
@Flags – You can specify value 0 or 1, 0 value will show user mapping to all databases and 1 will show the user mapping to current database only. Default value is 0
use master
go
exec sp_msloginmappings 'sa', 0

use master
go
exec sp_msloginmappings 'sa', 1

In case you want to run the sp_msloginmappings across multiple SQL Instance using either Central management server or powershell. You can use the below script.
create table #loginmappings(
LoginName nvarchar(128) NULL,
DBName nvarchar(128) NULL,
UserName nvarchar(128) NULL,
AliasName nvarchar(128) NULL
)
insert into #loginmappings
EXEC master..sp_msloginmappings
select * from #loginmappings
drop table #loginmappings