I would consider using gene expression signatures to classify samples (especially cancer subtypes but the same principles apply to other problems of this type) one of the classic problems of bioinformatics. Quite a lot of work has been done on methods to derive gene sets that provide good classification performance. This is slightly different from your problem since you already have a gene signature but it may still prove useful.
These methods will typically fit a model that selects a (small) number of genes from genome-wide expression data that distinguish between the cell types/conditions in question, i.e. they derive a gene signature. The resulting model then allows for the classification of new samples. I've had success using GeneRave for this purpose (but note that this was designed for microarray data, I haven't used it with RNA-seq data and don't know how well it holds up there). A more recent paper relating to this issue can be found here.
So how does that help you? One option would be to fit one of these classifiers to gene expression data for the genes you already know to obtain a model that can then be applied to new samples automatically.