Gas gauge seems stuck

I bought a 2008 pt cruiser 02/07/2018.  When I
drove off the lot the gas gauge read just under 3/4
of a tank. The next day I noticed the tank hadn't
moved despite driving about 30 miles in the city. I
went to the gas station and put in $10 to see what it
would do. It moved up to a little above 3/4.  Now it
seems to be stuck back at just below 3/4. The
digital display stayed at "199 miles to empty".
Today I went out and initiated the dash gauge
/lights test and the gas gauge moved fine during the
test. After the test it's back at just under 3/4 and the
digital display now reads "188 to empty".  How can
I tell if this is a gauge issue or a sending unit issue
without paying a mechanic a ton of money?