Letting agents I have found are in general lazy ******** : Ring the council and give them the full names of the people living in your properties.
As soon as the new tenancy agreement is signed make sure you get copies from the letting agent.
If the agent has done a credit check, try and get the DOBS of all the people living in your properties, as this is a great help in tracing.
ALWAYS encourage 3 month property visits..I rented 6 houses for 10yrs,and had to come down hard, I had drug dealers,pros and illegals renting rooms and playing up.
In the end I got a firm from London to deal with all the scum, then sold the lot.
Good Luck