I believe they are going to expand the law shortly for Baltimore so that all homes built before 1978 will need to be tested for lead paint. If you don't do it now I'm sure you will need to in a year or two.
When you do get it tested you have two routes. You can do the dust wipes for the full-risk reduction or use the XRF gun to test for lead-free certification. The dust wipes are easier to pass since you can have lead paint in the house but as long as there isn't a high reading of lead dust you will pass. The downside to it is that it costs a couple hundred bucks and you need to re-test at ever tenant changeover.
When testing for lead-free an XRF gun is used to determine lead paint concentrations on different surfaces. The inspector will be able to tell you exactly where the lead paint is. I do this and if there are any lead paint affected areas that are small and not difficult to remove, encapsulate, etc. I have them taken care of. The inspector will then come back and ensure the lead paint is gone. Once it is lead-free you get your cert. and no longer need to perform any further testing. This is much easier to do when the property is vacant.
Also I believe if a high-risk tenant decides to move in to your property you need to have it tested as well. I'm also not sure about the risk to you if you don't get tested. I doubt that you could be 100% free of risk by saying "I didn't know of any lead paint because I didn't have it tested". You should get legal advice on this or just contact the MDE lead paint group and they could probably tell you exactly what you should be doing.
Hope this helps - Scott