You want to find the quotas that have been configured for a security principal (i.e., user, group, or computer).
> dsquery quota -part <PartitionDN> -acct <PrincipalName>
The following command searches for quotas that have been assigned to the RALLENCORP\rallen user in the dc=rallencorp,dc=com partition:
> dsquery quota -part dc=rallencorp,dc=com -acct RALLENCORP\rallen
The dsquery solution will find only quotas that have been directly assigned to a security principal. The msDS-QuotaTrustee attribute on quota objects defines a SID that the quota applies to. The dsquery quota command will look up the SID for the specified account and match that against quota objects that reference that SID. Unfortunately, this doesn't quite show the whole picture. A user could have a quota assigned directly, which the dsquery command would show, but the user could also be part of one or more groups that have quotas assigned. These won't show up using dsquery.
A more robust solution would entail retrieving the tokenGroups attribute of the user, which contains a list of SIDs for all expanded group memberships, and then querying each of those groups to determine whether any of them have quotas assigned. This is actually the type of algorithm that is used to determine a user's effective quota, as shown in Recipe 15.17.
Recipe 15.13 for creating a quota