Is there any measurement I could perform after RT that allows me to check the efficiency that the procedure had? Nanodrop cannot be used as remnants of RNA and poly T primers mask measure, or?
Nanodrop may be fine for the volumes but as you rightly guessed it is not so easy to distinguish DNA from RNA (and primers). Qubit (invitrogen) is a sensitive, dye based quantification assay. You can use that for estimating cDNA yield. The kit contains different dyes for DNA, RNA and protein estimation.