When you say data warehouse do you mean a Kimbal Star/snowflake Schema?

Depending on the original developer you may find PK's but no FK's, although its not unheard of to have no PK's either (and rather shody IMHO).

In the case of missing PK's I suspect there will be a clustered/unique indexes on either the natural/business key columns and/or the surrogate key column.

In regards to FK's it depends on the type of DW load process, if its a full refresh (Ie Truncate and Reload every table) then you are unlikely to find FK's on the tables due to the extra effort needed to drop all the FK's prior to the clear out, and re initiate them after the load, as well as the headache associated with issues arising when RI is blown.

Ifs its an Incremental load then FK's are likely to be in place as theres no need to drop and recreate them as part of the ETL process, but they may be set up with NoCheck.