How much the rent should be?

[Paigow Chan]

I check out Zillow website and find out my rental property is $275 less.
Are their Zestimate for sale price and Rent Zestimate for rental accurate?
What other good resources to figure out if my rental property is in fair price?
My 3 years tenant (a family of 4) is good. The lease is due end of May and I believe they will continue to rent. Will it be too late to inform them $200 monthly increase? The lease doesn't mention when to increase the rent. What is the reasonable % for rent increase? Also, can we request for inside house inspection? It has been 3 years, I hope they maintain the house reasonable. The property is in Northern Virginia.