How can I check whether an URI is at subject or not

Virtuoso Universal Server Tips, Tricks, and HowTos
1

I’m wondering if there is any way of checking an URI appears at RDF subject in a graph quickly.
Might be the same as the triple pattern like, <URI> ?p ?o ., but I don’t need any values bound to ?p and ?o.

Thanks.

2

Virtuoso has an isIRI() function that can be used to determine if a triple value is an IRI.

3

Thanks, but what I’d like to know is whether an IRI is used as subject of a triple or not.
I assume that isIRI() cannot tell us this.