Lyndsey,
I drive by my tenants every other week if they haven't provided me any issues and at least once a week if I have any concerns. As for interior inspections I go inside on a formal basis twice a year, spring and fall when I change the smoke alarm and CO2 batteries. I do this so I know they're working and use it as a chance to have a look around. Also, like Robert, anytime I have a contractor over or work being done I'll meet them and have a look around inside.
The language is set up in my lease already, I can enter the property with 24 hours notice. As long as I give them that you're covered. Robert is right, you don't want to get too worried about the level of clean. Different people have different standards however if you see something that presents an issue (dirty dishes all over the place, trash not taken out regularly, etc.) you should mention it as a pest control measure.
I spent 5 years in the Marines so I've literally endured white glove room inspections (and conducted them), these walk through inspections aren't that! You're looking for obvious deficiencies and damage. My suggestion is to ditch the word inspection and pitch it as a repair walk. Let your tenants know in advance that you're there to help out and encourage them to have a list of things needed. This will let them know that you're serious about taking care of your investment and are not there to penalize them.
My .02 cents :)