Hi Sally, welcome to the forum. To test whether it is agate you can perform a scratch test. This will tell you the hardness of the mineral in question. In the case of agate, the hardness should be 7 Moh's, while glass would be softer at around 5.
For most enthusiasts I'd recommend investing in some basic Moh's scratch picks (sometimes called scratch pens), which have a selection of tips of various hardnesses (photo example below). You then attempt to mark a discrete part of the crystal with the pick - if it scratches then the mineral being tested is softer than, or equal to, the hardness value of the pick. If it doesn't scratch then the mineral is harder than the pick value. You can check what Moh's hardness value a mineral should be in the Crystal Guide app (it's listed as Hardness under the 'Geological Data' heading).
Alternatively, if you have some other minerals that you know are genuine, you can use these to perform the test in place of a dedicated scratch pick. The process is the same and you are just comparing whether the hardness of one mineral is greater or less than the other in order to work out the hardness of the mineral in question. It can require a fair amount of pressure while doing the testing.