Yep, tap on it. If it sounds like plastic soft, dull, it's acrylic. If it's hard like glass, it's glass. The edges will also look like plastic...
Generally they are built with different characteristics. Acrylic tanks will have the corners glued together with acrylic cement leaving a clean corner, whereby glass will have a thin layer of silicone between the glass and a bead of sealer about half an inch on either side of each edge all around the bottom and sides. Acrylic may have rounded corner in the front panel, glass will be a 90degree and unless rimless it'll have a large top plate (generally for strength) with cut out to access the inside. Glass will have a black plastic frame. The bottom of an acrylic tank will be flat so it can sit directly on any proper level surface and glass will have that black plastic frame like the top. Glass tanks will only have the botton pane tempered for obvipous reasons whereby acryilic will 100% acrylic all around.
Although newer tank are being design differently now this should give you a pretty good idea of the distinguishing characteristics typically found in most tanks...