If you click the header of one of the metadata columns in the Filter bar, you'll get a long list of the metadata fields that can be filtered. If the filed you are trying to filter isn't in that list, you may be able to construct a text search in the filter bar.
This doesn't solve my issue. I want to find all those photos where the "Source" field is empty. This is not possible with text search. Any other suggestions?
The only way to search the IPTC Source field in LR is to use the "Searchable IPTC" criterion:
This searches all the "searchable" IPTC fields (Adobe doesn't specific exactly which fields those are, but they do include Source).
However, you want to find photos whose Source field is empty. LR doesn't have an "is empty" criterion for most fields, but the normal way you'd work around that, e.g. for Title, is to do:
Title Doesn't Contain a b c ... x y z 0 1 2 3 4 5 6 7 8 9
Unfortunately, that workaround won't work in your case, since
Searchable IPTC Doesn't Contain a b c ... x y z 0 1 2 3 4 5 6 7 8 9
would only return those photos all of whose IPTC fields are empty. If there was a photo with a non-empty IPTC Headline and an empty Source, this criterion wouldn't return the photo.