Cut some photo paper, put it back in the holders, close the holders and take them outside for 5-10 minutes. Come back inside and develop the paper. If its fogged or you have lines near the edges (and you did things right) the holders will need work. If everything is fine, no problem.
When I vacuum my holders (and I do before loading them for every shoot), I always make sure to run the vacuum over the felt at the hinges . This may keep the felt from becoming too compressed and causing light leaks. It may not, but I do it anyway.