1. Check for seams. 
Glass from a mould often has seams, which show as lines in the finished product. The number of lines depends upon the shape of the finished article & construction of the mould. Depending upon the age of the mould (they wear with use) the pattern may be more or less sharp. In a faceted drop such as a 'crystal', the seam will run all the way around the edge like a ring for poured glass. Although there is nothing to stop pressed glass being cut subsequently the issue is usually: is this design cut or moulded?

2. Inspect the interior. 
In poured or pressed glass, the interior of the glass might have have slight dimples that mirror the exterior.

3. Examine the design. 
Hold the glass so that it reflects the light & look, particularly on facets, for fine striations from the tools used on cut glass. These are more obvious on older glass, where polishing was undertaken by hand. However, modern cut glass is chemically smoothed to remove all traces of cutting. This can leave as smooth a finish as good pressed glass. Also, on older cut glass there are often variations in the shape & spacing of the pattern. Not so in pressed glass, where the mould was an expensive item, usually made by craftsmen who perfected the design before it was used.