Try using Fast.com and checking your speed there. It is about the most bare bones speed tester there is. You then need to compare that to what you're paying your ISP for. If you're paying for 15 mbps and getting 5, then they need to check the line to the house and then the line to the modem/user. You generally get a percentage above what you're paying for, so if you're paying for 15 mbps, you should be getting closer to 20. I pay for 60 and get over 70.
The key here is that they must give you what you're paying for, so it is on them to correct the situation. Call them and they can do a total reset on their end, which can often improve your speeds. If you aren't getting what you're paying for, then they'll test your line and then your modem. It is on them to replace your line and/or modem from that point. Try calling them and asking for a reset, as it does seem to work
from time to time.