Looking for genes that are expressed the same in all the samples
I cannot think of a way without performing differential expression analysis. Since you don't have controls, what you could do is assigning some of the samples as controls.
Let's say you have samples A, B, C, D, E and F. You can perform differential expression analysis A, B, C (i.e. treatment) versus D, E (control). If you perform this for all possible combinations (that would be 5 choose 2 which is equal to 5 choose 3 if you decide to go for 3 vs 2 splits) and filter for genes which do not show up in any DE list you end up with the genes that are expressed similarly in al of these samples.