I was just about to say what Seminole posted, You are allow to show the property with proper notice to the current tenant. The tenant can not refuse your request to show the property and as someone else pointed out, you can also require them to keep the property in a show-able condition otherwise they may forfeit their security deposit. That one may depend on the contract you have with them.
I hate to say this, when you own rental property you just have to be an ruthless asshole and not care about people since they will screw you and blame you for trusting them. This is why I do not own rentals, I actually care about most people and care about what I own.