First, check the transmission fluid level and its condition. If it is low, look underneath for any leaks. It may have been topped off when you bought it, but leaked out as you drove. If it is still full, then check its color and odor.
Fresh transmission fluid will look a clear red or slightly orangish color. If it looks more pale red, clear, gray, or brown then it is due to be changed. If it has turned opaque, then it has been oxidized due to overuse...definitely time to change it!
Next is its odor. Transmission fluid doesn't smell great when fresh but it does have a distinctive odor compared to engine oil. Buy a quart and take a whiff to get familiar. Overused fluid will have a pungent, burnt smell to accompany its worn out color. Again, this is the sign of fluid left in too long...change it!
If this is your case, I will suggest you do NOT go and have your transmission power flushed in hopes of getting rid of all the bad fluid immediately. This procedure has been known to cause transmission failures shortly thereafter. Instead, have the tech change the fluid and in-pan filter. Drive the car for a few hundred miles, then have the fluid changed again. A few hundred more, and change it again.
Doing it this way allows any built up particles and varnish to slowly be removed or redissolved back into the fluid. Power flushing causes these particles to become dislodged en masse and pushed into the intricate valves, seals and passages inside.
Good luck and let us know how you fare.