To find your property lines, you should begin with checking your deed - it should contain a description (in words) of your property’s boundaries. If you do not have access to the deed, or the description is not sufficient to follow, you can look at the property survey (also known as a plat) which should show your property lines and measurements.
A final option, if the deed or property survey are not available or are unclear, is to hire a professional surveyor to do a land survey. They can measure and map your property so you have a clear understanding of your boundaries. The costs may vary depending on your location and project and you may want to collect estimates from several surveyors to find the one that best suits your needs. The surveyor does need to be licensed in Maryland and should carry professional liability insurance.