The Assemblathon 2 paper includes a variety of metrics for evaluating assembly quality.

Regarding N50, I suggest you read some introductory material, such as this.

This said, I am not a big fan of N50 as a quality metric for genome assemblies (or rather, as the UNIQUE quality metric). Several work has been performed on this issue and I suggest you to give a look to this paper.

You have to be very careful with individual metrics such as N50, which is why people generally publish a full table of metrics for their assembly. See previous (6 years ago!) discussion here: What Does The "N50" Mean? and How To Assess The Quality Of An Assembly? (Is There No Magic Formula?)

As genomax2 already mentioned in the comments, I would suggest looking into QUAST (Quality Assessment Tool for Genome Assemblies): http://quast.sourceforge.net/quast.html

It's a nice tool to visualize your assembly quality without worrying about specific metrics.

Also, I see you tagged the question as "rna-seq". In that case, all the metrics are going to be different than for genome assembly, since you are expecting thousands of relatively short transcripts as opposed to a few long chromosomes.