Here is a long form version of an answer that may give you some tools to solve this sort of problem on your own in the future.
First, when you are viewing file types from specific vendors, it is helpful to have their native viewer, if they offer it. In this case, the
.dxf file type is created by Autodesk. They offer a free viewer called DWG Trueview. Don't let the name fool you, it also reads and displays
Once you have this, a first step to determining the characteristics of a file like this is to open it and look around. Since you know that this is city blocks, it gives you a frame of reference for the scale of the features you are looking at. Look at the coordinates in the bottom left, and see what they are. See how quickly they change as you move the cursor. If they change faster, it is likely they represent a smaller unit of measure, possibly feet.
Here is a screenshot, that is zoomed in:
Compare the cursor location and the coordinates to this one, and see how much they changed:
From the amount they change, it is probably safe to assume that the units are in feet. The next step is to find a spatial reference that might match. The best place for that is here: Spatial Reference
Massachusetts and see what comes up.
Since you are just trying to get a baseline, you can just choose the first one that lists feet. A caveat to this is that you want to choose the newer datum, in this case, NAD 83 (North American Datum 1983). In this case, you are looking for the city of Boston, so it would be logical to choose one of the
Let's try the first one in the list: EPSG:2249: NAD83 / Massachusetts Mainland (ftUS)
If you open QGIS, and add the
dxf layer, it will ask you to select a coordinate reference system. If you enter the
EPSG code 2249 into the Filter box, you can see that it filters the list in the lower part, and the first one listed is the one you found above.
Now, you just need a reference layer to compare it to. Open Street Map is a great source for that. Their Wiki, tells you how to reference the data in QGIS.
Choose one of the tiles, and load it in. If it overlays properly, you know you found the correct coordinate system, and the correct units for your data. If it doesn't, then you can go back and try a different one.
In this case, you get a nice overlay:
The path to the solution all starts with looking at your data first, and trying to glean as much information out of it as you can. In this case, looking at the coordinates, then understanding the scale of the data, and the location all provided important clues to finding the appropriate spatial reference.