Well you can't tell for sure unless you pull the heads off, but there are some other things you can check to get an idea. Look at all the bolts holding the upper intake on. See if they have any markings on them that would indicate they were removed at one time. Also check all the clips on the wiring harness. See if any aren't clipped in place or are no longer attached to the harness. Also look at the connectors on the harness and look for any broken retaining tabs. After the 140K or so miles that the head gaskets usually go at, the clips become brittle and sometimes some of the tabs will break while removing them if you aren't careful. If you can put the car in the air, take a look at the bolts holding the y-pipe to the manifolds. See how rusty the bolts are, cause usually by the time head gaskets are needed, they have rusted so bad that they snap or have to be torched out, and sometimes they will be replaced with a nut and bolt instead of the studs.
Now all of these things aren't guarantees as to whether the head gaskets were ever done or not, but it will certainly give you a better idea. If you go over all those things, and don't find any broken or loose clips, no signs of bolts having been removed and reinstalled, no new hardware on the exhaust, etc. then most likely the head gaskets were never done. On the other hand, if you find all of these things, it still isn't a guarantee that they were done, but it would be an indication that they had, so maybe not worth collecting parts to do them again.
Of course, the "if it ain't broke, don't fix it" approach is still probably the best regardless of whether they were done or not, unless you plan on taking the car on some long trips and don't want to risk getting stranded.
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad