In a few weeks I'll take my check ride in an ASK21. All of my training
to date has been in a G103.

any advice?

Other posters have told you there isn't much difference and it won't
be a problem. They are probably all experienced pilots with many
types flown. I expect that you are not.

Don't take a check ride in a glider you have never flown. Get at least
3 takeoffs and landings in it to give yourself a fair chance on the
checkride.

Checkrides have very little to do with gliders, and everything
to do with the pilot. If flying a different glider makes you even
the slightest bit nervous, yeah, show up an hour early and get a
pattern tow at least.

Heck, I've personally never flown ANY two place aircraft
without flying it at least once with someone who has flown THAT
EXACT aircraft before. And I've hired test pilots three times to
demonstrate and fly single seaters before I flew them (I've flown
exactly three single-seat aircraft).

It only takes one pull of the wrong knob to end up surprised, still on
tow, but without a canopy...

How about asking the DPE? There is no law against his flying with
you for an hour of dual before the checkride. He's not the
one who signed you off, so there'd be no conflict. $75 for an hour
with a super experienced instructor right before your checkride?
Sounds like a good deal to me! If I were an examiner, I'd
be impressed by a cautious and methodical applicant...